The .450 Bushmaster is descended from the Thumper concept popularized by gun writer Jeff Cooper. Cooper was dissatisfied with the small-diameter bullet 5.56×45mm NATO (.223 Remington) of the AR-15, and envisioned a need for a large bore (.44 cal or greater) cartridge in a semi-automatic rifle to provide one-shot kills on big-game animals at 250 yards.[5] Inspired by this, LeGendre developed his .45 Professional cartridge in the early 2000's and built and delivered an AR-10 in .45 Professional to Cooper.[6][7]
After being sold the .45 Professional concept in 2007, Bushmaster requested the ammunition manufacturer Hornady to produce the .45 Professional cartridge for this project, but Hornady wanted to shorten the cartridge case and overall length to accommodate their 0.452 in. 250-grain pointed SST flex-tip bullet; plus .452 is a very common bullet size, being used by the .45 Colt, .45 ACP, .454 Casull, & .460 S&W Magnum, and the 250-grain SST was already a popular bullet for the .454 & .460.[5] Bushmaster and LeGendre approved the change from a 1.772 in. (45 mm) case and 2.362 in. (60 mm) overall length (OAL) to the now standard 1.700 in. (43.18 mm) case and 2.260 in. (57.40 mm) OAL. This permitted operation in the more abundant and popular AR-15 platform, versus the larger less common AR-10 platform. Also, a name change to ".450 Bushmaster" was approved.[8]
.450 Bushmaster single-stacks in magazines, unlike .223, which sits in two staggered columns.450 Bushmaster cartridges and components
The .450 Bushmaster makes use of .452 in. bullets because the lower impact velocities and energies would not adequately expand the heavier jacketed .458 in. bullets.[12] The cartridge is chambered in bolt-action rifles by Ruger, Savage, Mossberg, and Remington. Ruger also chambers their Ruger No. 1 single shot rifle, and their AR-556 MPR rifle in .450,[13] and Franklin Armory's XO-26 BFS-III equipped AR-15 pistol is chambered in .450.[14][15] As of 2022, Bushmaster Firearms is once again manufacturing rifles chambered in the .450, including their original fixed-stock Bushmaster 20" barrel AR;[16] the BOAR which also has a 20" barrel and features Magpul furniture including a UBR collapsible stock, pistol grip and M-LOK rail;[17] and the Bravo Zulu AR which features a 16" barrel, Magpul MOE Carbine Stock & MLOK rail, and Bushmaster proprietary accessors like the DM2S® 2-stage trigger and Snake Charmer®II muzzle brake; all three models come with a 5-round magazine.[18]
Ballistically, the .450 Bushmaster has a rather flat trajectory out to 200 yards, if the firearm is zeroed at 150 yards, the shooter can expect to see a rise of 2.2 inches at 100 yards (215 grain bullet), and a drop of 5.8 inches at 200 yards; although at this point it starts to drop off rather quickly, with the bullet dropping 32.8 inches at 300 yards.[19] The cartridge fits single-stacked in a standard AR-15 magazine with a single-stack follower. A 10-round AR-15 magazine body yields a four-round [20] magazine, a 20-round AR-15 magazine body yields a five- to seven-round [21] magazine, and a 30-round body yields a nine-round [22] magazine. As of 2025 Ammo Inc., Federal, Hornady, Remington, and Winchester manufacture ammunition for the rifle,[23] and Starline manufactures empty brass for handloading.[24]
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