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|Place of origin||United States|
|Designer||LeMAG Firearms LLC|
|Parent case||.284 Winchester|
|Case type||Rebated rim, straight wall|
|Bullet diameter||.452 in (11.5 mm)|
|Neck diameter||.4800 in (12.19 mm)|
|Base diameter||.5000 in (12.70 mm)|
|Rim diameter||.473 in (12.0 mm)|
|Rim thickness||.054 in (1.4 mm)|
|Case length||1.700 in (43.2 mm)|
|Overall length||2.260 in (57.4 mm)|
|Maximum pressure (SAAMI)||38,500 psi (265 MPa)|
|Test barrel length: 20 in (508 mm)|
Source(s): Hornady & Remington
The .450 Bushmaster is a rifle cartridge developed by Tim LeGendre of LeMag Firearms, and licensed to Bushmaster Firearms International. The .450 Bushmaster is designed to be used in standard M16s and AR-15s, using modified magazines and upper receiver assemblies.
The .450 Bushmaster is descended from the Thumper concept popularized by gun writer Jeff Cooper. Cooper was dissatisfied with the small-diameter 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. Inspired by this, LeGendre developed his .45 Professional cartridge,[when?] and later[when?] built and delivered an AR-15 in .45 Professional to Cooper.
Bushmaster requested[when?] 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. Bushmaster and LeGendre approved the change from a 1.772 in. (45 mm) case and 2.362 in. (60 mm) 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 AR-10 platform. Also, a name change to ".450 Bushmaster" was approved.
Loadings and ballistics
The .450 Bushmaster makes use of .452 in. bullets because of the lower impact velocities and energies would not adequately expand the heavier jacketed .458 in. bullets. The cartridge is chambered in bolt action rifles by Ruger, Savage, Mossberg, and Remington, Ruger's No. 1 single shot rifle, AR-15 rifles, as well as an AR-15 pistol by Franklin Armory.
Ballistically, the .450 Bushmaster has a rather flat trajectory out to 200 yards; if the firearm is zeroed at 150 yards, the user can expect to see a rise of 1.8 inches at 100 yards, zero at 150 yards, and a drop of 4.9 inches at 200 yards. 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  magazine, a 20-round AR-15 magazine body yields a five- to seven-round  magazine, and a 30-round body yields a nine-round  magazine. Hornady, Remington and Federal now manufacture ammunition for the rifle, and Starline manufactures empty brass for handloading.
- 11 mm caliber
- List of rifle cartridges
- Table of handgun and rifle cartridges
- .444 Marlin
- .458 SOCOM
- .50 Beowulf
- 12.7×55mm STs-130
- List of AR platform cartridges
- "CARTRIDGE and CHAMBER DRAWING - 450 BUSHMASTER" (PDF). SAAMI. p. 147. Archived from the original (PDF) on 2018-08-22. Retrieved 2018-08-22.
- PREMIER® ACCUTIP™ 450 BUSHMASTER Remington Arms Company
- Frank C. Barnes (5 October 2012). Cartridges of the World: A Complete Illustrated Reference for More Than 1,500 Cartridges. Gun Digest Books. pp. 97–. ISBN 1-4402-3059-5.
- "45 Cal .452 250 gr FTX® (450 Bushmaster)". Hornady. Archived from the original on 2017-07-10. Retrieved 2016-08-13.
- Pridgen, D.K. "FRANKLIN ARMORY XO-26 .450 BUSHMASTER". Tactical-Life Magazine. Retrieved 26 Feb 2014.