The .450 Bushmaster
By Chuck Hawks
The .450 Bushmaster was designed by the Hornady Manufacturing Company for Bushmaster Firearms International when the latter wanted a .45 caliber cartridge that would work through the short AR-15 action. The result of this collaboration is the .450 Bushmaster.
The Bushmaster is based on a sawed-off .284 Winchester case. It is a rimless, straight wall case with a rebated .473 inch diameter rim and a .500 inch diameter head. The case is 1.70 inches long. It uses .452 inch diameter bullets, as used in .45 caliber handguns, not standard .458 inch diameter rifle bullets. The loaded overall cartridge length is 2.250 inches and the case takes small rifle primers.
The .450 Bushmaster's MAP is 38,500 psi, considerably below that of the .223 Remington. However, the Bushmaster has much greater back thrust against the bolt face, so be careful not to exceed the SAAMI pressure limit.
Lacking a rim, shoulder or belt on which to headspace, the case must headspace on its mouth, like a typical autoloading pistol cartridge (.45 ACP, etc.). This prevents the use of a roll crimp, as used by most big bore rifle cartridges, to keep the bullet in place during recoil.
"It is important NOT to use a heavy roll crimp when loading bullets with cannelures, like the 240 and 300 grain XTP-MAG. Using too much roll crimp can allow the cartridge to travel too far into the chamber and not fire, or worse yet fire with too much headspace, which can create an unsafe condition."
This is a warning from the people who developed the cartridge, so take it to heart. As you can see, the limitations inherent in the AR-15 action necessarily resulted in a compromised cartridge design. The rebated rim, short case length, .452 inch (handgun) bullet diameter, poor bullet sectional density (SD), lack of a roll crimp and headspacing on the case mouth are all inherent design limitations that degrade the cartridges reliability, performance and (potentially) safety.
Typical bullets used in the .450 Bushmaster weigh between 240 grains (SD .168) and 300 grains (SD .210). Hornady designed a 250 grain FTX bullet (SD .175) without a cannelure specifically for the .450 Bushmaster. This is probably the bullet of choice for most purposes, despite its poor SD. Hornady made this bullet tougher than normal in an attempt to somewhat mitigate its inferior SD and get the bullet into the vitals of larger animals.
The importance of sectional density is something that has continued to escape the notice of most hunters, although it is difficult to understand why. Sectional density relates directly to penetration (a higher number being better) and normally a SD of .205 would be about the minimum for hunting Class 2 (deer size) game. A SD of around .250 is generally regarded as the minimum acceptable for Class 3 game (elk, moose, etc.) and a SD around .270 or greater is preferred.
.450 Bushmaster cartridges are factory loaded by Hornady and Remington, among the Big 4 US ammo makers. Remington offers a 260 grain AccuTip bullet (SD .183), while Hornady offers their 250 grain FTX bullet.
The latter claims a muzzle velocity of 2200 fps and muzzle energy of 2687 ft. lbs. from a 20 inch barrel. At 100 yards the numbers are 1835 fps and 1868 ft. lbs., and at 200 yards the remaining velocity is 1515 fps and the remaining energy is 1274 ft. lbs. The 2016 MSRP for a box of 20 Hornady .450 Bushmaster cartridges is $42.45, so the .450 is definitely a cartridge worth reloading.
Zero the Hornady factory load at 171 yards (i.e. +3 inches at 100 yards) to take advantage of its maximum point blank range (+/- 3 inches) and the trajectory looks like this (sight height 1.5 inches):
-1.5" at muzzle, +1.83" at 50 yards, +2.99" at 100 yards, +1.53" at 150 yards, -3.11" at 200 yards, -11.59" at 250 yards.
The .450 is about a 200 yard cartridge for Class 2 game and perhaps somewhat less for Class 3 game. The limiting factor is the short, fat, light for caliber bullet's poor sectional density. Take care to drive the bullet directly into the heart/lung area with a broadside shot, especially if hunting big animals like elk. Do not attempt raking shots.
The reloader with a .450 Bushmaster rifle can use just about any jacketed bullet intended for the .45 Colt revolver cartridge, as long is there is enough power to reliably operate the AR-15 rifle's autoloading action. For this reason, loads below the minimum published in the Hornady Handbook of Cartridge Reloading should be avoided.
However, for hunting big game, the 250 grain Hornady FTX is the obvious choice. It is the only .452 inch bullet Hornady recommends for large (Class 3) game in their .450 Bushmaster cartridge.
For reloaders, the Hornady 300 grain XTP-MAG bullet offers superior SD and is about as heavily constructed as .452 handgun bullets normally get. It is intended for use in .454 Casull and .460 S&W revolvers. However, Hornady recommends this bullet only for medium game. For short range Class 2 game hunting, I suspect it would knock the stuffing out of deer, hogs and black bear, much as does a 265 grain .444 Marlin bullet.
The technicians at Hornady reported that excellent results were obtained with Hodgdon Lil' Gun powder. Hornady reloading data shows that, using Hornady cases, Winchester WSR small rifle primers and the 250 grain FTX bullet, a minimum charge of 25.2 grains of Lil' Gun powder gives a MV of 1700 fps. The maximum charge of 37.9 grains gives a MV of 2200 fps, equal to the factory load velocity. A reasonable compromise for most woods hunting would seem to be 32.8 grains of Lil' Gun for a MV of 2000 fps.
The .450 is certainly a much better deer cartridge than the .223 Remington For use on Class 3 game, the .450 Bushmaster is probably about as good as any cartridge you can stuff into an AR-15 type rifle.
Note: An more in-depth article about the .450 Bushmaster can be found on the Rifle Cartridges page.
Copyright 2016 by Chuck Hawks. All rights reserved.