Originally chambered in .22 caliber, the AR-pattern rifle has seen widespread adaptation for a number of cartridges. Most are in the small- to- mid-caliber chamberings, but there are a few big-bore offerings in this once diminutive rifle.
The most popular big-bore cartridge available in an AR-15 is the .450 Bushmaster. It officially debuted in 2009, but its roots trace back to the wildcat .45 Professional developed by Tim LeGendre. He created the cartridge thanks in part to the “Thumper” concept espoused by Jeff Cooper. The goal was a cartridge capable of one-shot kills on big game up to 250 yards.
While the .450 Bushmaster is no Johnny-come-lately, its popularity is greater now than when it was introduced. The rise in demand can be attributed to a series of state laws making the cartridge legal for hunting whitetail deer. For instance, in 2016, Michigan began allowing rifle hunting in its southern counties. To be legal, rifles had to be chambered for straight-walled .35-caliber or greater cartridges with case lengths between 1.16 and 1.8 inches, giving the .450 Bushmaster a huge spike in sales. At the forefront of all this demand was Sturm, Ruger & Co.
For many years, Ruger has made an effort to make the .450 Bushmaster accessible to all shooters. First came the low-cost, bolt-action American, then the Cooper-inspired Scout and the timeless No. 1. Now Ruger is fielding the AR-556 MPR in the hard-hitting .450 Bushmaster.
Making It Work
Chambering an AR-15 in such a large cartridge entails a lot more than just installing a bigger barrel and opening up the bolt face. The bolt face on an AR chambered in .223 measures a scant .378 inch. The bolt face on a .450 Bushmaster bolt measures .473 inch. Opening up a bolt face that much, while still making it fit in a .223-sized barrel extension and receiver, leaves very little material around the case head when the cartridge is chambered.
The issue isn’t the pressure generated from firing the cartridge that causes the bolt head to rupture. Instead, thinning the amount of material around the case head reduces the amount of material holding the bolt lugs to the bolt body. Since there is a bit of residual chamber pressure when the bolt unlocks, binding force stresses the lugs and can lead to breakage. This is why Ruger spent a lot of time and resources choosing how they machine the .450 Bushmaster bolt, opting for a high-strength superalloy.
A quality AR-15 bolt is usually constructed from Carpenter 158 or 9310 steel. Carpenter 158 has been the preferred material since Eugene Stoner’s team selected it back in the late 1950s. Where 9310 offers a slight improvement over Carpenter 158, it’s still only appropriate for the smaller case heads, which leaves more bolt mass around the case head and additional material for bolt lugs.
Ruger is very tight-lipped about the composition of its super alloy, but one other manufacturer makes a similar product — the bolt alone sells for nearly $220. If you compare the price of the .450 Bushmaster to its 5.56mm counterpart, you’ll notice Ruger charges $200 more for the Bushmaster variant. This is due to the additional machining and material costs associated with manufacturing a bolt from this super alloy.
Given its nature, the mystery bolt material is very hard and dulls tooling quickly. However, it’s worth the price of admission to prevent a bolt lug from shearing at an inopportune moment. Ruger is currently the only manufacturer of a .450 Bushmaster AR that takes this highly inconvenient, but very important, manufacturing step, just to ensure the rifle doesn’t lose a lug.
This bolt should have no problem handling the 38,500 pounds per square inch (psi), which is the maximum SAAMI chamber pressure allowed for the .450 Bushmaster. (Most rifle cartridges have a max pressure between 55,000 and 64,000 psi.) The .450 Bushmaster’s low pressure is why the muzzle blast is relatively mild and why just about everyone who shoots one gets off the rifle and says, “That wasn’t so bad.”
The .450 Bushmaster is an excellent cartridge for hunting big game, but it is far from punishing. Guns & Ammo did all of its testing with polymer-tipped jacketed bullets that are ideal for deer, elk and moose. Hard-cast flat-nose (FN) bullets are available from Buffalo Bore and would be a good choice for bear or other dangerous game out to 150 yards. Even with the tame muzzle velocity of 1,700 feet per second (fps), the 360-grain bullet will still penetrate about 4 feet of bone and muscle. Think of the .450 Bushmaster as a lightly loaded .45-70, but available in a handy little semiautomatic platform.
The AR-556 MPR in .450 Bushmaster comes with a 15-inch, free-floating forend that surrounds an 18½-inch barrel. A carbine-length gas system is used on this rifle due to the low chamber pressure of the round. The long forend provides the shooter with plenty of real estate for his support hand in any positional shooting and lots of options when looking for field rests.
The barrel is hammer-forged with 5R rifling and a 1:16-inch twist with a corrosion-resistant nitride finish. Bottom line? This barrel should last forever. Hammer-forged barrels have work-hardened bores and, when combined with the low pressure from the .450 Bushmaster, throat erosion will occur very slowly. An owner would have to shoot thousands of dollars in ammunition through the barrel before it would wear out.
The Magpul MOE SL stock and MOE pistol grip are quality parts that are not typically found on factory guns in this price range. Magpul does a lot of destructive testing on its line and builds its stocks and grips to take abuse. If you’re overly-aggressive mortaring an AR to clear a stuck case, the buffer tube will likely deform before a polymer Magpul stock breaks.
Ruger manufactures its own trigger in this rifle called the Ruger Elite 452, a two-stage model with a pull weight of about 4½ pounds. The trigger has a crisp let-off and short overtravel. It is equally at home shooting tiny groups or rapidly dumping five rounds into whatever is trying to eat you.
Spending a day at the range with this rifle is not exactly pleasant, but it’s far from painful. Shooting steel all day isn’t going to be a popular pastime with the .450 Bushmaster, but walking through bear country with one would be far less troubling than without it. Every time the rifle fires, just over one ton of muzzle energy flies downrange. That will solve a lot of life’s problems when judiciously applied. Thanks to the detachable magazine, there can always be five rounds on tap.
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