More Punch for the AR-15

 

© Anthony G Williams

 

This article first appeared in the May 2005 issue of The Cartridge Researcher, the bulletin of the European Cartridge Research Association

Updated January 2013

 

It is now four decades since the USA adopted the ArmaLite AR-15 as its new light rifle, under the designation M16. In that time, it has been adopted by a large number of armed forces and it is rivalled only by the Kalashnikov family for the title of the most widely used assault rifle.

One of the characteristics of the AR-15 is its modular construction. It is divided into ‘upper’ (barrel and action) and ‘lower’ (the rest) assemblies, which makes it particularly easy to rebarrel the gun to take cartridges very different from the 5.56 x 45 it was designed for.  Needless to say, American gunmakers have taken advantage of this to offer AR-15 based guns chambered for a wide range of different cartridges. Some, like the 6.8 x 43 Remington SPC and 6.5 x 38 Grendel, are intended to provide an overall improvement on the oft-criticised performance of the 5.56 x 45 NATO. Others are more specialised and this article describes four of them, all designed for a similar purpose: the .450 Bushmaster, .458 SOCOM, .499 Leitner-Wise and .50 Beowulf.

As the calibres indicate, these are big-bore cartridges of considerable power. However, they are all designed to remain within the overall length of the 5.56 x 45 cartridge so that they can function in the AR-15 action. Muzzle velocities of these short rounds are therefore lower than most rifle cartridges in such large calibres, but they still provide considerable short-range hitting power.

 

The .450 Bushmaster is the newest of the four. It was developed by Hornady for the Bushmaster company and is based on the .284 Winchester case, so has a slightly rebated rim. Calibre and case length measurements are 11.5 x 43 RB, body diameter is 12.7 mm, rim diameter 11.9 mm, and bullet diameter 0.452 inches (11.5 mm). The initial loading is a Hornady polymer-tipped FTX "Flex Tip" bullet weighing 250 grains. Muzzle velocity is 2,200 fps (670 m/s) from a 20 inch (508 mm) barrel, 1,700 fps (520 m/s) from a 16 inch (406 mm). From a 20 inch barrel, the velocity falls to 1,840 fps (560 m/s) at 100 yards (90 m), 1,525 fps (465 m/s) at 200 yards (180 m) and 1,265 fps (386 m/s) at 300 yards (270 m). Muzzle energy is 2,700 ft/lbs (3,635 Joules) from a 20 inch barrel.

Hornady is the only manufacturer to date. Special magazines have to be used.

 

The .458 SOCOM (Special Operations Command) ammunition is manufactured by Cor-Bon for the Teppo Jutsu company who developed the SOCOM. The 11.4 x 40 RB cartridge case has a rim diameter of 12.0 mm and a body diameter of 13.6 mm, with a bullet diameter of .458 inches (11.6 mm). Unlike the other rounds described here, it is slightly bottlenecked. It is offered with bullet weights ranging from 300 to 600 grains (19.5 to 39 g) at muzzle velocities from subsonic to 2,000 fps (610 m/s). Muzzle energy is 2,680 ft/lbs (3,610 J) with the 300 grain bullet at 2,000 fps.

It is claimed that it isn’t even necessary to change magazines for the new round: the standard 30-round 5.56 mm magazine will hold 10 rounds of .458 SOCOM ammunition, whereas the 20-round magazine will hold 7, and the USA 40-round version up to 15 rounds. Ammunition is made by Load-X and SBR as well as Cor-Bon. More details of the SOCOM are HERE.

 

The .499 LWR (Leitner-Wise Rifle) measures 12.5 x 44 RB with a rim diameter of 11.2 mm, a body of 13.3 mm and a bullet diameter of .50 inches (12.7mm). The starting point for the development of this cartridge was the .50 AE pistol round with a reduced-diameter rim, but the case design eventually selected is not only longer but also has a different internal construction. The pointed hollow-point 300 grain (19.5 g) bullet has a lead-free frangible construction and is claimed to have good ballistics. It is fired at a muzzle velocity of 2,180 fps (650 m/s) and at 200 yards (180 m) is still travelling at 1,650 fps (500 m/s). Muzzle energy is 3,055 ft lbs (4,120 J).

Purpose-designed magazines are used because feeding problems were found to arise with the standard M16 items. The LWR company has changed hands and direction over the past few years (the initials now stand for Land Warfare Resource corporation), and .499 guns and ammunition are no longer offered, but the round is included here as it may still be encountered and brass cases are made by Silver State.

 

The .50 Beowulf is manufactured by Alexander Arms (also responsible for the 6.5 mm Grendel). It measures 12.5 x 42 RB and has an 11.2 mm diameter rim with a 13.5 mm body and a bullet diameter of .50 inches (12.7 mm). Various loadings are offered with bullet weights from 290 grains (19 g) to 400 grains (26 g). Muzzle velocities vary from 2,000 fps (610 m/s) for the 290 grain bullet to 1,800 fps (550 m/s) for the 400 grain from a 16 inch (406 mm) barrel, or from 2,050 (625 m/s) for a 21 grain bullet and 1,900 fps (580 m/s) for the 400 grain from a 24 inch (610 mm) barrel. Remaining velocities at 200 yards (180 m) from the 24 inch barrel are 1,210 fps / 370 m/s (325 grain / 21.1 g) and 1,280 fps / 390 m/s (400 grain / 25.9 g). Muzzle energies are in the 2,620-2,915 ft/lbs (3,530-3,930 J) range from a 16 inch barrel, 3,040-3,240 ft/lbs (4,100-4,370 J) from a 24 inch barrel.

Ammunition is offered by Load-X as well as Alexander Arms. The magazine is a slightly modified standard 5.56 mm M16 magazine, and larger capacity magazines can be used with the Beowulf by carefully bending the feed lips to reliably feed the larger cartridge.  Twelve of the .50 calibre cartridges can fit into a 30-round M16 magazine.

 

It will be noted that both the Beowulf and the Leitner-Wise use a rim diameter the same as the 7.62 x 39 (AK 47) round, while the Bushmaster and the SOCOM use the same rim as the 7.62 x 51 NATO. All of them, however, have a larger body diameter in order to achieve the calibre required or, in the case of the SOCOM, to provide enough propellant capacity.

 

 

Any of these cartridges provides dramatic short-range barrier penetration and hitting power which accounts for the interest in them among law enforcement organisations and special forces. The .499 LWR was tested by the US Coast Guard, but without result. In its heavy-bullet loadings the .458 SOCOM (and potentially the other two) also provides the basis for a hard-hitting subsonic cartridge suitable for use in silenced weapons. And all whilst retaining the compact dimensions, light weight and selective fire option of the M16 assault rifle.

 

 

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