A 'PRIMER' ON CARTRIDGES

 

 

Anthony G Williams

 

with thanks to Nelson H. Lawry and others for comments

 

Adapted from 'Rapid Fire: the Development of Automatic Cannon, Heavy Machine Guns and their Ammunition for Armies Navies and Air Forces', with additional information from 'Assault Rifle: the Development of the Military Rifle and its Ammunition since World War 2' .

 

The purpose of this article is to answer some of the questions which are always asked by people as they become interested in cartridges and want to learn about them. It consists of a basic guide to cartridges including their design, naming conventions and projectile types.  The emphasis is on military cartridges, but some civilian terms and examples are included.

 

Having absorbed this article, the next stage should be to read the ones on Basic Ballistics and The Search for High Velocity. Many of the larger cartridges are illustrated in the Ammunition Photo Gallery and data are provided in the Ammunition Data Tables. Other terms which you don't recognise may be identified from the Glossary. All of these are on this website.

 

What are cartridges and how are they described?

 

A cartridge is a unit or round of ammunition consisting of a projectile (which may be a bullet, a shell or shot), propellant to thrust the projectile from the gun, a primer to ignite the propellant and a cartridge case to hold it all together. 

 

Cartridges are usually described primarily by their calibre (or caliber in the USA); this is the diameter of the projectile, or something close to it. Measurements of the calibre may be given in inches or millimeters. So a '.30 calibre' cartridge (sometimes written .30") is one in which the bullet measures about 0.30 inches in diameter; the same cartridge described in metric terms would be a '7.62 mm' round. Whether inch or metric designations are used is a matter of choice by the designer; generally speaking, European cartridges use metric designations while the UK and USA have traditionally used inch measurements, but the use of metric terms is spreading.

 

I have said that the calibre is approximately the same as the bullet diameter because two different measurements can be used: the diameter of the bullet, or of the 'bore' (the inside diameter of the gun barrel). Gun barrels, with the exception of shotguns and a few specialized tank guns, are 'rifled '; they have spiral grooves cut into the bore to make the projectile spin, greatly improving accuracy. The bullet diameter therefore needs to be in between the diameter of the bore and that of the rifling, so that it is gripped firmly.

 

This is part of the reason for different calibres being used to describe cartridges of the same bullet diameter; .308 and .300 cartridges actually use the same bullets - .308 inch is the bullet measurement, .300 the bore. In addition, cartridges are sometimes given calibre designations which differ slightly from these, simply to make cartridge identification easier (or to put it another way, to distinguish them from rival cartridges).

 

The actual measurements of many common rifle cartridges are shown below. These are averages, since manufacturing tolerances vary:

 

Bullet diameter mm/inches

Bore diameter mm/inches

Cartridge designations

5.60/.221

5.5/.217

5.45 mm Russian

5.69/.224

5.56/.219

5.56 mm, 5.6 mm, .218, 219, .22, .220, .221, .222, .223, .224, .225, .226

6.2/.244

6.0/.236

6 mm, .240, .243, .244

6.53/.257

6.35/.25

6.35 mm, .25, .250, .257

6.71/.264

6.5/.256

6.5 mm, .256, .260, .264

7.04/.277

6.86/.270

6.8 mm, .270, .277

7.21/.284

7.0/.276

7 mm, .275, .276, .280, .284

7.82/.308

7.62/.300

7.5 mm, 7.62 mm, 7.63 mm, 7.65 mm, .30, .300, .307, .308

7.90/.311

7.7/.303

7.62 mm (Russian), 7.65 mm (Belgian), 7.7 mm, 8mm Swedish, .303, .311

8.20/.323

7.92/.312

7.92 mm, 8 mm (most - but not all)

8.59/.338

8.38/.330

8.58 mm, .33, .330, .338, .340

12.9 / .510

12.7 / .50

.50, 12.7 mm

 

Pistol cartridges are less confusing as there tend to be fewer of them for a given calibre. The following table gives the main examples:

 

Bullet diameter mm/inches

Bore diameter mm/inches

Cartridge designations

5.33/.210

 

5.45 mm

6.38/.251

 6.2/.244

.25 ACP

7.82/.308

 7.63/.3005

7.63 mm Mauser / 7.62mm Tokarev

7.85/.309

 7.73/.3045

.32 ACP, .32 Colt & S&W

9.0/.355

 8.83/.3475

9 mm (nearly all), .356, .380 Auto

9.1/.358

 8.85/.3485

.38 (most), .357

9.2/.363

 

9 mm (Makarov)

10.16/.400

 

10 mm, .40, .400

10.4/.410

 10.02/.3945

.41, .410, .38-40

10.9/.429

 10.62/.4185

.44

11.5/.452

 11.25/.443

.45 ACP

 

Even with such a range of calibre designations, confusion can still exist between many different cartridges of the same calibre, so additional terms may be used. These can broadly be grouped as follows:

 

The length of the cartridge case. This has been used in inch measurements in the past (especially old British large-calibre hunting rounds, like the .450 3 inch) but is standard in metric measurements, e.g. the 7 x 57. Note that this is the length of the case only, not the overall length of the case plus bullet (which may vary depending on the type of bullet loaded). These metric measurements are now standard for nearly all military ammunition, including in the UK and USA.

 

Other cartridge designations are many and varied, and include the following:

 

- the weight of propellant, in grains (.30-30: usually from the black-powder era, but in this example nitro powder)

- the date of introduction of the cartridge (.30-06; introduced in 1906)

- the velocity of the bullet (.250-3000; 3,000 fps)

- the name of the designer or firm (.25 Remington, .465" Holland & Holland)

- the name of a sponsor (STW Shooting Times Westerner)

- the name of the country for which it was intended, or which used it (.44 Russian)

- the name of a gun particularly associated with it (7.62 mm AK/M)

- a description of the propellant type (Nitro)

- an indication of increased velocity through a light bullet (Express) or a bigger case for more propellant (Magnum)

- a name to express a characteristic of the round (.220 Swift)

- a name to impress (.577 Tyrannosaur)

- mention of the cartridge from which it was developed (.450/400" or 6.5 mm/.284: note that in the former British example, the actual calibre comes last, while in the latter American case it comes first: and combines metric and inch measurements!).

 

These types of description can be combined, e.g. in old British rounds such as the .450/400" Magnum Nitro Express 3".

 

This calibre designation system applies to rifle and pistol cartridges and modern military ammunition in general. However, there are other systems for different types of weapons. Shotguns are universally described according to their 'bore' or 'gauge'; this is based on the weight of a round lead ball which fits the barrel compared with a weight of one pound. The term 'twelve bore' means that twelve of these balls will together weigh one pound. So in this instance, the smaller the number, the bigger the calibre; a ten bore has a larger calibre than a twelve bore. This system was also used to describe old, black-powder, large-calibre hunting rifles. The one common exception to this is the .410 shotgun, which is the calibre in inches (10.4 mm). Shotgun cartridges are further measured in length, by inches; the longer they are for a given gauge, the more pellets or propellant they carry.

 

The calibres of different shotgun gauges are as follows:

 

1 bore

2 bore

4 bore

8 bore

10 bore

12 bore

16 bore

20 bore

28 bore

1.669"

42.4mm

1.325"

33.7mm

1.052"

26.7mm

.835"

21.2mm

.775"

19.7mm

.729"

18.5mm

.662"

16.8mm

.615"

15.6mm

.550"

14mm

 

In the past, the weight of the projectile was also used to measure artillery pieces, particularly in the UK. Guns were described as 'pounders' (which may be shortened to Pdr or Pr). This referred to the notional weight in pounds of the projectile fired by the gun, so the '25 pdr' gun/howitzer normally fired a shell weighing 25 pounds or 11.4 kg. Originally, the basis for measurement was a round steel shot, but as rifling allowed the use of elongated steel shells, this relationship broke down. The British persevered with this system until after the Second World War, adjusting the 'pounder' measurement accordingly, which meant that some guns of the same calibre were given different poundage measurements to denote the weights of the shells fired. The calibres associated with common poundages follow:

 

37mm

40mm

47mm

57mm

76.2mm

(3 inch)

83.5mm

88mm

127mm

(5 inch)

1 pdr

1 pdr

2 pdr

3 pdr

6 pdr

12 pdr

13 pdr

17 pdr

20 pdr

25 pdr

60 pdr

 

In British use, 'poundage' measurements were used alongside 'inch' and even metric measurements. For example, in World War 2 the British Army had the 25 pdr and 5.5 inch field guns and the 95 mm howitzer, all developed in much the same period.

 

Another method of designation associated with large-calibre guns (but sometimes applied to the ammunition which they fired) is the 'calibre length', which measures the length of the barrel as a multiple of the calibre. So a 40 mm L/70 Bofors gun has a barrel 40 x 70 = 2,800 mm long. The ammunition it fires is the 40 x 365R but it is often known as the 40mm L/70. To confuse matters further, there are different practices in measuring barrel length which can give slightly different L numbers for the same gun.

 

Finally, the term calibre is sometimes used rather loosely to mean more than just the diameter of the projectile. For example, the term '7.62 mm NATO calibre' means the 7.62 x 51, while a term such as '39 calibre' may be used to describe a 155 mm artillery piece with a barrel of that length.

 

The Cartridge Case

 

The cartridge case has two major functions.  First, it holds together all of the active components of the cartridge - the projectile, propellant and primer - in a waterproof container which is rugged enough to withstand rough handling, especially in automatic weapons.  Second, when the gun is fired, the cartridge case is expanded by the pressure against the walls of the firing chamber, forming a gas-tight seal which prevents any propellant gas from seeping back into the gun mechanism - and possibly into the firer's face.

 

The second function explains why brass is still the most popular material for small-arms cartridge cases despite its weight and cost.  Standard cartridge brass (70% copper, 30% zinc) has exactly the right characteristics in that it expands instantly to form a seal without being split open.  It may also be reloaded easily several times, which makes it attractive to the frequent shooter.

 

Nowadays steel is normally used in larger cannon, particularly linear-action guns with a high rate of fire, because its extra strength is better at coping with the violent treatment the cartridge is given by the loading mechanism.  It is also slightly lighter and cheaper.  However, it requires more protection against corrosion and, being less resilient, forms a less perfect seal on firing.  Steel cases have been used in military small arms, especially by Germany in WW2, and are still the norm in Russia and China, but they have never been popular for commercial use.

 

Light (aluminium) alloy has even more attractions (it saves between 25-38% of the total weight of a cartridge) but for a long time it proved very difficult to achieve satisfactory results with this material and it has only recently come into common use.  The first and most notable example is the 30x173 ammunition for the General Electric GAU-8/A fitted to the A-10 aircraft.  This has such a large ammunition capacity that a weight saving of over 270kg is achieved compared with the Oerlikon KCA's dimensionally similar steel-cased rounds (the actual difference in empty case weight being 143g instead of 350g).  Light alloy ammunition in this calibre has also been developed for the 30x113B used in the M230 Chain Gun. Attempts to develop light-alloy cases for small arms have been generally unsuccessful, although they are used in some disposable pistol ammunition.

 

Plastics have seen little use so far, except in shotgun cartridges (which operate at a much lower pressure than rifles) where it has replaced cardboard, but have been developed for military rifle/MG rounds and may enter service in the near future. Such cases always have metal bases as the plastic is not strong enough to resist tearing by the extractor.

 

Finally, some rounds have combustible cases, designed to burn up in the chamber, while others are 'caseless' the propellant is formed into a solid block. With the exception of large artillery and tank gun rounds, these have not so far entered service.

 

The earliest cartridge cases were made in more than one piece, typically an iron head to which was fixed a tube made from coiled brass.  These were very much an interim measure, soon replaced by the one piece case which has been universal since the latter part of the 19th century.  This is formed by discs, punched from a thick sheet of metal, being repeatedly "drawn", that is mechanically pressed into a hollow shape, until the desired case shape has been achieved.  At the same time, information such as the cartridge designation, maker and year of manufacture are stamped into the head.  The final operation is usually the cutting of the extractor groove close to the head.

 

Cartridge case design is a science in itself.  There are certain basic requirements - the case must locate the projectile in precisely the correct position in the firing chamber (with the exception of advanced primer ignition - API - blowback guns which fire as the case is moving forwards), it must be easy to extract once fired and it must function well in automatic weapons - but there are various methods of achieving these aims.

 

Before proceeding further it is well to be clear about the nomenclature of cartridge cases.  The basic elements - head, rim and neck - are shown HERE (unfortunately these terms are rather confusing in that the head is as far from the neck as possible so it is now often referred to as the base).  Cases usually have an identifying headstamp around the primer pocket, which contain information about the cartridge.  Rimless cases need an extractor groove and "bottleneck" cases also have a shoulder, where the case is reduced in diameter down to the neck in cartridges in which the calibre is significantly smaller than the case diameter.  Some cases have a belt - an annular projection -  just above the extractor groove.

 

The information provided by the headstamps varies considerably. In civilian cartridges this usually consists of the cartridge designation (.460 Weatherby), or the name of the ammunition manufacturer if that is different (.303 K = Kynoch). In military cases the date of manufacture is usually present (1943 or 43) together with letters which indicate the manufacturer. Other information may also be included, and the subject is so complex that there are whole books devoted to it.

 

In modern small-arms cartridges the inside neck diameter is made fractionally smaller than the bullet, thereby ensuring that the bullet is gripped firmly until it is fired.  In the larger calibres a more authoritative means of securing the projectile is normally required and this is provided by crimping; the case neck is pressed into recesses in the projectile, either all around the circumference or at several points on it.

 

Accurate location in the firing chamber is essential to the ignition of the primer; the cartridge needs to enter the chamber, but not too far or the firing pin (or electrical contact) will not touch the primer.  It is also important in the internal ballistics of the gun, as the projectile must be in the same place, relative to the conical section leading to the rifling, to achieve consistent pressure characteristics.  The distance between the face of the bolt and the part of the chamber which locates the cartridge is known as the "headspace".

 

The earliest method of achieving accurate location was by means of a rim around the head of the case.  This remains outside the chamber or in a recess so that it is flush with the breech face.  The rim also provides something for the extractor to hook on to in order to pull the fired case from the chamber.  Rimmed cases are still satisfactory except that the rim can complicate ammunition feed arrangements in automatic weapons.  In spring-loaded magazines the rims can foul each other if improperly loaded, while in belt-fed guns it is usually necessary for the cartridge to be withdrawn backwards from the belt before being pushed forwards into the chamber.  This design has therefore largely been replaced for military purposes, although it is still encountered, for example in some Russian small arms and in larger cannon from 40 mm upwards.

 

By far the most common military case type is the rimless, in which the rim is reduced to the same diameter as the case so that the cartridges in magazines can be stacked on top of each other or be pushed forward from ammunition belts without risk of jamming.  To give the extractor something to hook on to, an extractor groove is cut into the case.  There have been a few examples of semi-rimmed (or semi-rimless) cases, in which the rim is only fractionally wider than the case and is combined with a small extractor groove, but these have not been a particularly successful compromise. 

 

Rimless cases need a method of accurate location in the chamber and this is normally achieved by using a bottleneck case, in which the headspace is determined by the contact between the shoulder of the case and the matching part of the chamber.  With straight or tapered cases this becomes more difficult to achieve so such cases often have a projecting belt just in front of the extractor groove to provide positive location.

 

A few cases have seen service with rebated rims, i.e. with a rim of smaller diameter than the case.  This was essential in API blowback cannon which fired before the cartridge was fully chambered, as it allowed the bolt and its extractor claw to follow the case into an extended or hooded chamber.  This ensured that the cartridge was fully supported at the instant of firing. Rebated cases are sometimes used in rifle cartridges in which a wider case is wanted while retaining the standard bolt head diameter.

 

Blowback weapons do not strictly require case extractors to function, as the fired case is pushed out of the chamber by gas pressure.  Some early designs for such weapons accordingly used cartridges without rims or extractor grooves.  This was soon proved to be a bad idea, because the only way of unloading an unfired cartridge was to poke a stick down the barrel.

 

Letters may be added to the cartridge name to denote some variation from the usual rimless head.  The 20 mm Becker is known as the 20 x 70RB because it has a rebated rim of smaller diameter than the case (sometimes expressed as RR); the 30mm Aden is called the 30 x 113B because the case is belted (has a raised strip around the case just above the extractor groove); the rimmed 20 mm ShVAK was known as the 20 x 99R; the semi-rimmed 1.1" US Naval round as the 28 x 199SRExamples of these different case types are shown HERE.

 

In artillery calibres, a distinction is made between fixed, semi-fixed and separated (or separate-loading) ammunition. In fixed ammunition, the projectile is firmly fixed into the case. In semi-fixed, the case and projectile are loaded as one unit but can be separated to permit varying the propellant quantity to meet the circumstances. In separated ammunition, the projectile is loaded separately from the case to ease handling.

 

By now, it should be realized that stories about how some military cartridges could be fired in guns chambered for different ones are almost entirely inaccurate. There is no way that cartridges like the .30-06, 7.62 x 54R, 7.62 x 51 or 7.62 x 39 could be fired in the same weapon without at least changing the barrel (and probably doing a lot more in the case of automatic guns). The only partial exception is where cartridges have been given different names in different countries: so the .303" British will chamber in World War 2 Italian and Japanese 7.7 x 56R weapons and vice versa because it is in fact the same cartridge. However, even then there is no guarantee that automatic weapons will function properly (or that all rounds will be safe to fire in all weapons), as the pressure characteristics of the cartridge loadings may be different.

 

In commercial rounds, straight-cased rimmed cartridges of different lengths (but otherwise similar) can often be fired in the same gun; but only if it is chambered for the longer cartridge. So for example shotguns chambered for the 12 gauge 3" will also chamber 12 gauge 2" ammunition, and .357" Magnum guns will chamber the .38" Special, but not the other way round. Finally, some guns chambered for 'improved' versions of rifle cartridges can often fire the 'standard' rounds; doing so typically expands the case to the 'improved' shape, a process known as 'fire-forming'.

 

Projectile Types: Small Arms and Commercial

 

The projectile is what the weapon is all about.  Everything else - the rest of the cartridge and the gun itself - is concerned with ensuring that the projectile hits the target at the desired velocity.  The design of the projectile is therefore the most crucial aspect of ammunition design.

 

The basic type of military bullet is known as 'ball' ammunition. This was named after the round lead balls which were the standard small arms projectiles until the nineteenth century.  The name is still applied to standard rifle ammunition, in which the bullet consists of a jacket (originally copper, now a variety of alloys) normally enclosing a lead core.  Lead is still used because it has a high density (which helps the ballistics) and is relatively cheap, although steel is sometimes used as alternative. It is enclosed in a jacket of harder material primarily to prevent it from being torn apart by the rifling as it passes up the barrel, because this not only ruins the accuracy of the bullet, it also leaves an unwelcome deposit in the bore. Fully jacketed bullets also avoid any accusations of illegality as (unlike soft-point hunting rounds) they do not expand on impact to maximise the wounding effect. The jacket sometimes has a cannelure  - a roughened band running around the center to provide a location for the crimp.

 

Bullets for rifles usually have pointed noses in order to reduce air resistance and thereby reduce the rate at which they lose velocity. Sharp-pointed ones were developed first in Germany and are still sometimes called 'spitzer' bullets. Their centre of gravity is behind their mid-point, so that left to themselves they would naturally travel base-first. To prevent this, they are spun by the barrel's rifling, which provides stability. Many bullets are also given tapered rather than flat bases (also known as 'boat-tailed' or 'streamlined' bullets) as this further reduces the aerodynamic drag, particularly at subsonic velocities.

 

Other types of projectile used in military ammunition include armour piercing, or AP. In AP bullets, much of the lead core is replaced by hard steel, or in some cases tungsten carbide which is harder and more dense for greater effectiveness (at higher cost). They are therefore sometimes called APHC, for armour piercing hard core, to distinguish them from other AP types.

 

Experimental military rifle rounds have been made to fire flechettes (little arrows) , which are thin, fin-stabilised projectiles. They are normally fired from smooth-bored barrels. Various rifles have been developed to fire such rounds, but none has yet made it into service.

 

Tracer bullets are also sometimes used, but more often in light MGs. These are hollow at the base to contain a chemical which burns in flight to reveal the trajectory of the bullet and indicate whether the shooting is on target. However, the bullet tends to be lighter (and gets lighter still as the chemical burns) and the gas emitted by the tracer also affects the bullet drag, so no tracer can provide exactly the same trajectory as a ball or AP round.

 

Incendiary and even HE projectiles have been used in rifle calibre ammunition, most notably from WW1 to WW2 in aircraft guns. However, such small projectiles are barely able to contain enough chemicals to make the effort worthwhile.  The heavy machine gun is really the smallest calibre in which incendiary shells are common, with HE shells being more common in cannon (20+mm) calibres. 

 

 Military rifle bullets other than the standard ball round are often identified by different coloured tips. Various colour schemes have been used in different places and at different times, but a black tip usually indicates an AP bullet, a red tip a tracer. Other colours are much less predictable; e.g. the USA used a green tip to indicate the 7.62 x 51 M198 Duplex ball loading, but it is now used for the M855 SAP, which is their current standard 5.56 x 45 round.

 

Commercial rifle bullets are almost always made to expand in order to increase the size of the wound channel when hunting game. This is generally achieved by leaving the lead core exposed at the tip of the bullet, but hollow-points, with a cavity in the nose, have been used instead. The only hunting bullets designed not to expand are those made for deep penetration on large African game like buffalo and elephant; these are also round-nosed to ensure that they penetrate nose-first (pointed bullets generally tumble on impact).

 

Pistol bullets have traditionally been of lead alloy, and that is still common for low-velocity revolvers. However, in self-loading ('automatic') pistols, the need for the cartridge to feed smoothly from the magazine to the chamber, coupled with the generally higher velocity of the weapons, led to the use of jacketed round-nose bullets. It is common to load hollow-points for greater expansion, but these are illegal in military use.

 

Shotgun cartridges do of course normally fire a quantity of metal shot (spherical pellets) with the aim of improving the hit probability against the flying birds which are the usual targets. The size of the individual pellets varies according to the  size of the target. Shot loadings are also sometimes used in pistol cartridges, but they are less effective partly because the rifled barrel disturbs the shot pattern (which is why shotguns have smoothbore barrels). Shotguns may also fire solid slugs for use against large game; originally just a round lead ball, they are now shaped (often with spiral grooves on the outside to encourage spinning) and some are saboted; a smaller-calibre slug in contained within a plastic sabot or sleeve which falls away at the muzzle.

 

Projectile Types: HMGs and Cannon

 

In larger military calibres, projectiles are also available in a wide variety of types, even ignoring those intended for practice or drill purposes which are usually made to look different from standard rounds.  Service ammunition can be classified into four broad categories - ball, incendiary (I), high-explosive (HE) and armour-piercing (AP) - any of which may also be equipped with tracers (T) so that the gunner can observe the accuracy of his shooting.  Other common designations are SAP (semi-armour-piercing), TP (target practice), SD (self destruct), APDS (armour-piercing discarding-sabot) and APFSDS (armour-piercing fin-stabilised discarding-sabot).  The terms HV (high or hyper velocity) and SV (super velocity) are sometimes used to designate lightweight projectiles (usually AP).  Some manufacturers use their own designations for proprietary types of projectiles. 

 

Cannon projectiles are usually painted according to their type for rapid identification purposes (in HMG projectiles it is normally just the tip that is painted).  Colour schemes have varied between nations and military blocs and within such groupings over time, so this is not the place to attempt a complete listing.  However, AP projectiles are nowadays commonly painted black and practice ones blue.  Reds and yellows frequently indicate chemical contents, either HE, incendiary or tracers.  Combined projectiles often have bands of colour to indicate their particular mix of characteristics, so projectiles can be quite colourful.

 

The method of construction of projectiles varies considerably.  In calibres of up to 15 mm it is common to use all-enclosing soft metal jackets as with rifle calibres, designed to be easily engraved by the barrel rifling, with incendiary and AP as well as ball rounds.  Larger calibres use steel projectile bodies fitted with separate driving bands (US: rotating bands) of greater than projectile diameter, which are gripped by the rifling in order to spin the projectile.  Originally driving bands were made of copper (still favoured by the Russians) but this is not strong enough to withstand the shearing effect of the rifling at very high velocities, so soft iron driving bands are used on most Western high-velocity cannon.  The shell bodies are often made slightly smaller than the bore in order to reduce friction, and will then have a bourrelet a section made slightly wider to fit the bore near the nose to keep the shell centred in the bore.

 

After much experimentation US designers have perfected plastic driving bands, first successfully introduced in the 1970s in 30 x 173 calibre for the 30mm GAU-8/A aircraft gun.  The advantage of plastic-banded projectiles is that barrel wear is only one-third that caused by metal driving bands.  Plastic and iron bands also avoid the problem of "coppering"; the coating of the bore with copper which has to be cleared from time to time with a "decoppering" round. 

 

Light-alloy projectile bodies are usually used in APCR (armour-piercing, composite, rigid) shot, in which only the hard central core penetrates the target, the light-alloy body breaking up on impact.  Sub-calibre projectiles such as APDS and ADFSDS are nowadays commonly encased in plastic sabots. It should be noted in passing that when projectiles exceed a certain length (around five to six times the calibre) it is no longer feasible to stabilise them by spinning, so they are fitted with fins instead, like an arrow. They are then best fired from a smooth-bored rather than rifled barrel. APFSDS can be fired from rifled barrels, but the sabots are usually fitted with slip rings to reduce the spin imparted to the projectile.

 

The simplest form of cannon AP projectile consists of a pointed steel shot - the military term "shot" (from the cannon ball era) meaning a solid or very thick-walled projectile, normally without any explosive content (although there were some exceptions, mainly in 19th Century usage when the term 'shot' could be used for APHE).  The steel will be hardened, at least at the point, and the shot will have a driving band.  The best AP performance is obtained with a rather blunt point, so the shot is sometimes given a "ballistic cap"; a pointed nose cone usually made out of light alloy or thin steel in order to minimise the added weight.  Some projectiles were also given a blunt cap of soft steel as this improved penetration against face-hardened armour, leading to designations such as APC (armour piercing, capped) and APCBC (armour piercing, capped, ballistic capped).

 

This simple type of AP shot was usual in automatic cannon until after the Second World War, when it began to be replaced by more sophisticated composite projectiles, such as APCR, APDS and APFSDS, described in The Search for High Velocity.  Heavy machine guns have tended to follow a slightly different route, retaining the cupro-nickel bullet jacket of the ball rounds but inserting a hardened steel or tungsten AP core to achieve the desired effect. One exception is the SLAP (saboted light armour projectile) APDS round produced for the .50 Browning.

 

The term "shell" is again of ancient origin and obvious meaning; a hard outer casing protecting something more vulnerable inside, in this case a high explosive or incendiary compound.  In larger artillery calibres, shells can contain a wide variety of materials including smoke to obscure visibility, starshell for illumination, chemicals, various antipersonnel rounds, and anti-tank sub-munitions (some capable of homing onto their targets).

 

Cannon shells were originally made by drilling out a cavity of the appropriate shape in a steel shot to take the desired explosive or incendiary compound.  This led to a relatively small capacity in smaller calibres, however, so during the Second World War, German technicians perfected the mine shell (Minengeschoss or M-Geschoss) which was made by stamping and drawing the shell from a thick disc of metal in the same way in which cartridge cases are made.  This resulted in a shell with thin but strong walls, with a far larger capacity.  It also led to a lighter shell overall (steel being much heavier than explosives), permitting a higher muzzle velocity at the expense of range (as lighter shells slow down more quickly), which made this innovation particularly suitable for aircraft cannon.

 

Anti-personnel shells in larger calibres have included Shrapnel, canister and beehive shells. Shrapnel (named after their 18th Century British inventor) consisted in its later, developed version of a shell filled with lead balls and with a small bursting charge in the base, ignited by a time fuze. The fuze was set to burst the shell open in mid-air, just before arriving at the target, so that it delivered a shower of lead balls. This was very effective against exposed infantry and cavalry so was the most popular field artillery ammunition before WW1, but it fell out of favour during that conflict as it was ineffective against trenches. The last recorded use of shrapnel shells was in WW2, although the modern CC and AHEAD rounds have similarities (see below). Nowadays, the term 'shrapnel' is loosely applied to any shell fragments. Canister rounds consist of a lead, steel or tungsten alloy balls contained within a thin case. When fired, the case bursts open immediately after leaving the muzzle, sending a cone-shaped 'shotgun blast' of balls with considerable short-range effect against groups of enemy soldiers. The origin of the ammunition is ancient, but it is still used in tank guns. The 'beehive' artillery shells contained a large number of flechettes and the shell was burst open by a fuze, again providing a spread of anti-personnel sub-projectiles. These were used by the USA in Vietnam.

 

One specialised form of HE projectile is the HEAT (high explosive anti-tank) or shaped charge shell.  In this, there is a cone-shaped hollow, lined with metal, left at the front of the shell.  An instant-acting fuze detonates the HE on the surface of the target.  The explosion compresses the metal lining to form an elongated high-velocity jet whose tip can be travelling at 8,000-9,000 metres per second, with considerable armour-piercing capability.  This type of projectile is standard in modern infantry anti-tank weapons and is used in some larger cannon.  It was experimentally used in the Second World War in ammunition for German aircraft guns, but has not been generally used in smaller calibres because the effectiveness is largely determined by projectile diameter and is also lessened by being spun by barrel rifling.  A rotation of only 150 revolutions per second is sufficient to reduce the penetration by 50%, yet a cannon shell will typically rotate at considerably more. 

 

In larger calibres, this has been resolved by firing fin-stabilised shells from a smooth-bored barrel (which is why APFSDS and HEAT are common 'partners' in modern, smooth-bored tank guns). In the Obus 'G' ammunition (named after the inventor, Gessner) for the French 105mm rifled tank gun, this problem was reduced by mounting the shaped charge on ball bearings within the case, greatly reducing the spin imparted to the explosive section, but this is not feasible with small-calibre ammunition.

 

However, the problem has been tackled by careful design of the shape of the HE cone and metal lining - generally a flatter cone is needed - and HEAT ammunition has quite recently become available for automatic cannon.  Known as the M789 HEDP (HE Dual Purpose - anti-personnel as well as anti-armour), it is the standard type of ammunition used in the 30mm M230 Chain Gun fitted to the AH-64 Apache attack helicopter, in which the angle of the rifling has also been reduced to the minimum level needed to achieve stability.  The M789 is capable of penetrating 50mm of steel as well as having a 4m lethal radius against unprotected personnel.

 

An alternative to HE and HEAT in larger calibers is HESH (HE squash head), also known as HEP (HE Plastic). This is a base-fuzed shell with a thin shell body designed to be squashed flat on impact, forming a pancake of HE which is then detonated by the fuze. This is very effective against certain targets as it sends shock-waves through the material being attacked, spalling off high-velocity fragments, but is less common now.

 

It should be noted that military terminology is not always logical.  All projectiles for the 20mm Hispano, including AP, were officially referred to as "shells" by the RAF.

 

It is usual for projectiles to combine characteristics in order to maximise the effect on the target. The term "AP shell" (which should properly be APHE) is used to describe an essentially armour-piercing projectile which also contains some high explosive (usually only a very small percentage of the projectile weight), intended to detonate once the armour has been penetrated.  It is generally only worth doing this in larger calibres, although national practices have varied.  In World War 2, for example, British AP shot for tank/anti-tank guns contained no HE, while the German equivalents did. 

 

More of a compromise is the SAP or SAPHE (semi-armour piercing HE) shell, which is similar to an APHE shell but has thinner walls to provide more space for explosives, thereby losing some AP performance.  Because of the necessity for a hard point to both APHE and SAPHE shells, the fuzes are fitted into the base.

 

A common form of combination adds a tracer element to an HE, incendiary or AP projectile.  These usually consist of a pyrotechnic element in the base of the projectile which burns brightly, enabling the trajectory to be followed and the aim corrected as required.  These are obviously more effective at night so daytime tracers sometimes utilise a smoke trail.  It is usual for the tracers to burn without emitting light for the first part of their travel ("dark trace"), partly to avoid blinding the gunner and partly to conceal the precise location of the gun.  An originally unintended side-effect of tracers is psychological;  the sight of a stream of tracers heading towards an attacking aircraft can distract the pilot.

 

Ammunition designations now include "HEI" (high explosive and incendiary) and "API-T" (armour-piercing incendiary tracer).  It is possible for all of the above types to be combined in one multi-purpose projectile but there is obviously the risk that it will not be very effective in any of its tasks.  The Norwegian firm of Raufoss has been particularly successful in developing Multipurpose (MP) ammunition which combines a chemical impact fuze with incendiary and HE elements - and even, in the .50" calibre version known as the NM140, some AP capability. 

 

Most projectiles are variants or combinations of those already described.  "Multiball" rounds have been developed for heavy machine guns as well as infantry rifles.  As the name suggests, two or more short projectiles are stacked on top of each other in the cartridge case and all fired at once, with the aim of increasing the hit probability against infantry.  These are also known as duplex or triplex loadings, when they have two or three projectiles respectively.  The problem is that, being light for their calibre, the projectiles lose velocity and range rather quickly. 

 

A development of this idea is the "salvo-squeezebore".  This consists of a number of funnel-shaped projectiles stacked on top of each other and held together by a plastic sheath.  When fired down a taper-bore barrel the projectiles separate and are simultaneously squeezed to a smaller calibre.  Each cartridge fired therefore results in a stream of small, high-velocity projectiles.  Although extensively tested in .50" calibre (12.7mm) by the Americans, who were attracted by its potential for short-range defence against ambushes, it was not adopted.

 

Some projectiles are designed to break up into a hail of sub-projectiles, either on or immediately before impact with the target.  The former are called "frangible", leading to the "F" designation.  Some of these have been developed for training, to ensure that the projectile breaks up on impact instead of ricocheting a long way into the distance (or, in the case of air-to-ground firing, back off the ground to hit the firing aircraft; a surprisingly high risk).  Other uses are more warlike, with the projectiles being intended to break up after penetration to cause more damage within the target.   This has also been applied to the APDS principle to create FAPDS, which has a useful capability against aircraft and light armour so has recently become very popular for Anti-Aircraft guns in 23-30mm calibre.  A special version of this is NWM De Kruithoorn's FMPDS (frangible missile-piercing discarding sabot) which is loaded into the 30x173 cartridges used in the Goalkeeper naval anti-missile system. For aircraft use, in which saboted ammunition cannot safely be used for fear of the discarded sabots being sucked into the engines, FAP has been developed which is increasingly popular. A new rival for this is PELE (Penetrator with enhanced lateral effectivness) which also contains no chemicals and is designed to fragment on impact.

 

A unique light cannon variant currently applied to Russian aircraft ammunition for ground attack purposes is the ME (Multi-Element) or CC (cargo carrying) projectile.  This contains subprojectiles which are discharged from the body of the projectile at a set distance after firing in order to saturate the target area.  It is designed to cause significant damage to such targets as aircraft parked in the open, but would also be highly dangerous to any personnel in the area.  The version for the 23x115 cartridge carries 24 x 2g subprojectiles, that for the 30mm guns 28 x 3.5g.  The 30x165 loading discharges at between 1,100 and 1,800m after firing, the subprojectiles forming a cone with an angle of 8.  Total projectile weights and muzzle velocities are similar to that of standard ammunition.

 

A more sophisticated approach is represented by the Oerlikon AHEAD (Advanced Hit Efficiency And Destruction) or KETF (Kinetic Energy Time Fuzed) anti-aircraft/anti-missile ammunition initially offered in 35x228 calibre.  The shell is timed to detonate 25m before it reaches the target, creating a 5m wide pattern of 152  3.3g tungsten sub-projectiles travelling at extremely high velocity due to a combination of the carrier projectile velocity and the force with which the sub-projectiles are ejected.  Antipersonnel versions of KETF are made in 30mm and 35mm calibres for used by AFVs.

 

A rival to AHEAD is the Bofors PFHE (pre-fragmented high explosive) shell available in 40mm and 57mm calibres as a component of the 3P system (prefragmented, programmable, proximity fuzed).  This differs in being an HE shell lined (in 40mm calibre) with 650 tungsten pellets, with detonation normally triggered by a proximity fuze.  The effective radius of the exploding 40mm shell is up to 7m against aircraft and 3m against sea-skimming anti-ship missiles.

 

Rocket-assisted projectiles (RAPs) are used in artillery ammunition to extend the range but as the space required for the rocket reduces the HE capacity by about 50% they are not used in smaller-calibre weapons.  However, an AP RAP (with the propellant wrapped around the penetrator) was experimentally developed for the 30mm GAU-8/A aircraft gun in the 1970s and demonstrated the same penetration at 1,800m as the standard APCR could achieve at 1,200m.

 

Not all projectiles are designed for offence; some shells have contents designed to defend aircraft against missile attack.  Some postwar Soviet aircraft cannon are available with chaff-dispensing anti-radar ammunition (PRL-23 in 23mm) and even infra-red decoy projectiles (IK-23).  The 23x115 version of the IR decoy round ignites about one second after firing and burns for some four seconds.  The 30x155B chaff dispenser projectile produces a cloud of radar-obscuring particles which grows from an initial 7-9m2 to a maximum of 14-18m2 in area.

 

Fuzes

 

The triggering of incendiary compounds might not be too critical as they burn for a noticeable, albeit short, period of time.  HE shells, however, have to be detonated at precisely the correct instant; and for this, a fuze is needed.

 

Every fuze has two distinct requirements.  First, it must reliably detonate the explosive filling at the precise instant required; this might be the instant it hits the target, a fraction of a second later, at a preset point of its flight, or when it detects the target is close enough (many fuzes are capable of being set to achieve more than one of these).  Secondly, it must be completely safe when being handled (or mishandled) and fired.

 

It is not easy to achieve these aims and fuzes are often intricate devices built with the precision of a Swiss watch.  They therefore tend to be expensive, which is why they are most common in cannon calibres where the cost can be justified by the effect.

 

Not all contact fuzes are mechanical.  Some, particularly in smaller calibres, use chemicals which detonate on impact.  These are far simpler in theory but very difficult to get right in practice, because there are no mechanical safety devices.  The characteristics of the fuze compound and its metal exterior therefore have to be extremely carefully judged to ensure the necessary combination of reliable detonation with safety when mishandled.  Put simply, the shell must never detonate when dropped point-down onto a concrete floor, but must always detonate when it hits a target at high velocity.  Raufoss Multipurpose ammunition uses such fuzes.

 

The vast majority of fuzes are fitted to the points of projectiles.  However, some types of shell make use of base fuzes, as in the case of the SAPHE shells already mentioned.

 

There is the obvious risk that shells fired at an attacking aircraft might carry on to detonate on landing some distance away, possibly among friends.  Contact and proximity-fuzed shells are therefore often fitted with a self-destruct device which detonates the shell after a certain period of time, when it can be presumed to have missed the target.  Sometimes this is a function of the fuze, but a simpler system used in smaller calibres is for the propellant to ignite a separate chemical fuze in the base of the shell; a reversion to the original form of time fuze.  This may also be achieved by the final stage of  a tracer burn.

 

Propellants

 

The earliest machine gun cartridges were designed to use black powder (gunpowder) as a propellant.  This is a simple mixture of saltpetre, sulphur and charcoal, and has been used for centuries.  It is not ideal as a propellant, however, because of its inefficient burning characteristics and the large quantities of smoke and other residues generated.

 

Improvements in chemical science in the late nineteenth century led to the development of smokeless powders, the first of these entering service in France in the late 1880s.  Other nations soon followed suit, with the British version being known as "cordite" because it was extruded in fine strings resembling cords, which were bundled together for insertion into the cartridge case. 

 

There are three different types of smokeless propellant, known as single-base, double-base and triple-base.  All are based on nitro-cellulose with nitro-glycerine (or equivalent) and nitro-guanidine as other major components in the more complex versions, triple-base including all three.  Cordite is a double-base type. 

 

The increasing complexity of the propellants is due to the constant search for the ideal combination of characteristics.  These include maximum power for a high muzzle velocity, moderate pressure and temperature to minimise the stresses and erosive effects on the gun, as little fouling and corrosion as possible, and a minimum of smoke and flash at the muzzle. Many of these desiderata are mutually exclusive so each propellant is a compromise.  Particular propellants may be optimised for special purposes, e.g. 'flashless' propellant for night-fighting, which may lose some performance in order to avoid any muzzle flash. As with other chemical ammunition components propellants must be insensitive to rough treatment, provide consistent performance over a wide range of climatic environments and be tolerant of storage for long periods in poor conditions.

 

The performance of a given cartridge depends upon the maximum pressure which the gun is designed to accept.  Pistols and shotguns, for example, are usually only intended to work at relatively low pressures while rifles, machine guns and cannon are generally much stronger, so cartridges for them can be loaded to higher pressures and therefore velocities.  Not all guns are the same, however, and some can take much higher pressures than others.  This is usually determined by the strength of the mechanism which locks the bolt or breechblock to the breech at the instant of firing.

 

Propellants today are generally prepared in the form of grains or small pellets.  The precise chemical composition, and the size and shape of the grains, will vary from one cartridge to another in order to provide the power and pressure characteristics suited to the gun.

 

Primers

 

Primers are older than metallic cartridges.  The first percussion primer was developed early in the nineteenth century and this invention soon replaced the flintlock in military service.  It consisted of a small copper cap containing fulminate of mercury, which produced an intense flash when struck a sharp blow from a hammer or firing pin and thereby ignited the propellant charge.

 

Many of the early metallic cartridges departed from this arrangement in that the priming compound was contained within the rim of the cartridge case, and fired by the impact of the hammer on the rim.  This proved inconvenient because it required the rim to be thin and soft, and also meant that the used case could not be reloaded.  The rimfire principle  therefore soon fell from favour except in the small inexpensive cartridges still used for target shooting. Another variation was pinfire, recognizable by the metal pin sticking out from then side of the case.

 

Rimfires were superseded by the return of the separate percussion cap, now housed centrally in a cavity, known as the primer pocket, in the head of the case (the cartridges therefore being called "centrefire").  It was so arranged that when struck, the primer fired into the propellant via a central hole or holes in the cavity.  Once fired, the primer could be knocked out of the case which could then be reloaded.  In larger calibres, the primer ignites an intermediate igniter charge.

 

Little has changed in primer design since then.  Fulminate of mercury has been replaced by less corrosive compounds and the precise details of the design of the percussion cap have been refined.  The push-fit of the primer in the case, typical of small arms, is supplemented by more positive means of holding it in place in larger calibres, and European cannon primers are often screwed into the case, apparently because this makes it easier to replace the propellant (the first element of ammunition to deteriorate after long storage).  The only major departure has been the replacement in some cases of percussion ignition with electric.

 

Electric ignition is achieved by passing an electric current from the gun through a special primer which is heated to ignition point virtually instantaneously.  It was first used in automatic weapons in Germany to facilitate the synchronisation of aircraft guns designed to fire through the propeller disc of a single-engined fighter.  Previously, this had involved a complex mechanical or electromechanical linkage between the propeller and the gun firing mechanism to ensure that the gun fired only between the propeller blades.  It was obviously simpler to design a direct electrical link between the propeller and the cartridge.  Ironically, it came into service in World War 2 just before the need for it disappeared because of the advent of jet engines.

 

Electric priming does have other advantages.  It simplifies gun design because the firing pin and its associated spring and mechanism for releasing, withdrawing and recocking the pin are replaced by a simple electrical contact.  However, it obviously requires a reliable supply of electricity and is therefore most suited to vehicle-mounted applications.

 

It is usual for cartridges to be designed either for percussion or electric ignition but there are some cases where a particular cartridge has been available in both versions.  They are difficult to tell apart visually, since only the internal design of the primer is different.  However, in electric ignition the primer has to be insulated from the rest of the case and this insulation is sometimes visible.  Needless to say, percussion and electric versions of the same cartridge have to be used in the appropriate gun as they are not interchangeable.  The best known examples are the 15 x 96 and 20 x 82 rounds for the MG 151, the 20 x 128 Oerlikon and the 30 x 165 Russian (which uses electrical priming for the naval and aircraft weapons, percussion for the AFV guns).  There are others; the percussion primed 20 x 110 widely used in the HS 404 and related guns was produced in an electrically-primed version for the postwar American M24.

 

Other Issues

 

There is often debate about the distinctions between 'machine gun' (MG), heavy machine gun' (HMG) and automatic cannon. There are no universal rules, and naming practices have varied in different countries and at different times. However, it is now generally accepted that MGs fire rifle-calibre ammunition (5.45 7.92 mm), HMGs are chambered for 12.7 15 mm rounds and cannon for 20+ mm cartridges. As far as projectiles are concerned, the generally accepted convention has been that MGs and HMGs are designed to fire jacketed ball, incendiary and AP bullets while cannon usually fire steel HE shells or specialised AP shot.  However, there have always been exceptions to this so it cannot be relied on.

 

It is often argued that there are legal limitations on the types and calibres of weapon which can be fired at people. There is some truth in this, but there is also much misinformation. In the middle of that century explosive projectiles were introduced in small arms.  These were felt to be inhumane when used against soldiers, so an international convention was held which led to the St. Petersburg Declaration of 1868, limiting their use to artillery projectiles weighing at least 400g (just under 1 lb).  At the time, this equated to a calibre of 37 mm so this became an international standard which was honoured for the next half century.  During the First World War, incendiary and explosive bullets were developed in rifle calibres for use by aircraft, and this situation was accepted after the war (although they were still supposed to be fired at other aircraft and vehicles, not people).

 

The Hague Convention of 1907 banned bullets calculated to cause unnecessary suffering, which effectively excluded the kind of expanding bullets (soft points and hollow points) used in hunting. However, there are no restrictions based purely on calibre; the oft-repeated story that .50 calibre guns may not be fired at people is a myth.

 

It may also be worth adding a note on the infamous dum-dum bullets. These were developed at the end of the 19th Century in the Indian arsenal at Dum Dum (then a part of the British Empire) to rectify the observed ineffectiveness of the new round-nosed .303 bullets. They were made by removing the tip of the jacket to expose the lead core, and sometimes hollowing-out the tip, to encourage expansion. At first these efforts were unofficial, but after some abortive attempts  the .303 Mark V cartridge was formally adopted by the British in 1899.This featured a deep hollow-point (like some hunting bullets of the period), but was later discarded as it was judged to be in contravention of the Hague Convention. The term 'dum-dum' now seems to be used to describe any bullet which has the nose unofficially modified to improve expansion.

 

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