Introduced by Bushmaster and Hornady in late 2007, the .450 Bushmaster was an attempt to improve the ballistic performance of AR-15 rifles. Launching a .45-caliber, 250-grain bullet at 2,200 fps is, of course, a significant performance step up compared to the 5.56 NATO’s relatively tiny 55-grain bullet exiting the barrel at 3,200 fps. At the time the .450 Bushmaster was introduced, not many folks hunted with an AR-style rifle.
Hornady was the only source for factory-loaded ammo at the time, but Remington and Federal later offered it. The round was slow to gain popularity and looked to be on a path to obsolescence, but a fortunate significant event occurred that saved the .450 Bushmaster from obscurity.
A few years ago, several states in the upper Midwest changed their hunting regulations. They decreed a straight-wall metallic cartridge larger than a specified caliber would be allowed to hunt whitetails. A dealer in Michigan convinced Ruger that adapting the American Ranch bolt-action rifle to chamber the .450 Bushmaster would be an excellent marketing move. Ruger has since shipped thousands of American Ranch rifles and recently introduced a second version of the AR-556 MPR autoloader chambered for the round.
I decided to investigate the performance of this big round and learned a few things about handloading it along the way. The .450 Bushmaster’s straight-wall case is slightly tapered, and the sizer die body is machined tool steel without a no-lube-required carbide or nitride insert. When reloading most rifle cartridges, I typically apply Redding case wax sparingly with my fingertips. With the .450 Bushmaster, I soon discovered each case needed just a light swipe of case lube to ensure the sizer die would reshape it properly without it getting stuck. A light application of Hornady One Shot spray lube or a swipe of Redding case wax worked well. Both lubes are easily removed after sizing by wiping with a cloth or tumbling in corncob media.
Although the .450 Bushmaster is a rifle cartridge, it is topped with pistol-size bullets of 0.452-inch diameter. And while there are many .45-caliber pistol bullets available, very few are constructed to accommodate the .450 Bushmaster’s velocities. Hornady’s 250-grain Flex Tip bullet was purposely designed for the .450 Bushmaster, and the Speer 300-grain Deep Curl bullet that’s designed for the .454 Casull is also suitable. I also included a prototype 260-grain Speer bullet that’s still in development.
The .450 Bushmaster’s case capacity is significantly greater than that of the 5.56 NATO/.223 Remington, but the cartridge uses Small Rifle primers to ignite the greater propellant charges. The .450 Bushmaster’s MAP (maximum average pressure) of 38,500 psi is only slightly greater than typical big-bore magnum handgun pressures, so it’s not surprising that it’s loaded with similar burn rate propellants. Since I would be firing half of the test loads in an AR, I chose to use CCI #41 mil-spec Small Rifle primers.
The Hornady die set includes a case mouth expander and a taper crimp die. When using either one, the operative phrase is “just a little is likely just enough.” You must expand the case mouth so the bullet heel will just seat and align itself properly when it’s inserted. Likewise, after seating the bullet to the desired cartridge overall length, a slight case mouth flare is evident. The taper crimp die should be adjusted to just barely iron the case mouth so it’s no longer expanded at all. There’s no shoulder or rim, so the round headspaces on the case mouth. Measurements indicated the case mouth diameter was typically reduced by a couple thousandths of an inch max.
I’ve never had to trim any other cartridge with a slightly tapered, straight-wall case, so I didn’t bother to check case length when reloading the fired brass two or three times. That was a lesson learned the hard way because it can be exciting when executing a GI-style extraction to clear a jam! You must check case length and trim cases when necessary.
Using brand-new Ruger American Ranch and AR-556 rifles to test-fire factory-loaded ammo and my handloads, I found the Remington and Federal factory loads fired in the AR delivered similar 1.75-MOA to 2.0-MOA accuracy. However, my handloads weren’t quite as accurate. While velocities often matched or exceeded the comparable factory-ammo results, standard deviations were greater. However, the loads in both rifles were powerful and accurate enough to take down the next whitetail you encounter at ranges up to a couple hundred yards.