The AR 15 comes in nearly any configuration you can think of. What started as a 5.56 caliber rifle has slowly shifted to a rainbow’s worth of calibers. This included a number of big-bore calibers that take the AR 15 from a .223 caliber weapon to a .45 caliber rifle. Today we are looking at the .450 Bushmaster cartridge, one of the more successful big-bore AR 15 calibers. The cartridge has a fascinating history that traces its lineage all the way back to Jeff Cooper.


History of the .450 Bushmaster

Colonel Jeff Cooper was a Marine, hunter, and one of the most influential firearm instructors of the 20th century. He came up with numerous concepts, including the famed Scout rifle, but he also dabbled with a concept known as the “Thumper.”

Col. Cooper wasn’t satisfied with the power of the little 5.56 and envisioned a large bore projectile in a semi-auto platform. Cooper wanted a gun that could provide a one-shot kill against big game animal at 250 yards or so. This concept was what guided Tim LeGendre of LeMag Firearms to develop the .450 Bushmaster.

dead hog AR 15
(Courtesy Michael E. Cumpston)

LeGendre took the casing from a .284 Winchester round and a .452 projectile to design the 45 Professional. Originally, he was going to produce a cartridge known as the .45 Professional, and this round had a slightly longer case. The round was licensed to Bushmaster at the time, and the round would be chambered in an AR-10. Bushmaster was a big name in ARs at the time, and their backing helped Tim LeGendre take the concept from the drawing board to the real world.

Bushmaster and LeGendre approached Hornady to professionally load the round. The 60mm case was trimmed to 45mm at the behest of Hornady, who wished to load the round with their .452 250-grain Flex Tip SST projectile. This reduction in length allowed the cartridge to be chambered in the much more common AR 15.

The round dropped the .45 Professional name and became known as the 450 Bushmaster.


The .450 Bushmaster in Action

The 250 gain .452 caliber round certainly fits in its role as a thumper of a cartridge. It’s a big heavy bullet that strikes a good balance between size and effective range. Unlike heavier big AR cartridges like the .458 SOCOM, the .450 Bushmaster has a respective max range of 250 yards or so. Compared to a 5.56 rifle, you might say that’s not very far. However, it’s respectable for such a big and heavy projectile backed by such a short case length. If it was magnum-sized, it would go further but wouldn’t fit in an AR-15 either. Plus, most hunting isn’t done at ranges longer than 250 yards, especially for big game.

Bushmaster rifle
The OG Bushmaster rifle.

When the round hits, you know it. It transfers 2,700 foot-pounds of energy on target and penetrates deep. The big bullet breaks through bone, tears through hide, muscle, and flesh to deliver an expanding projectile to the lungs and heart of your prey.

The .450 Bushmaster works with standard AR-15 lower receivers and parts. You’ll need a new upper and a proprietary magazine. Sadly, you can’t use standard AR 15 magazines and must use specific .450 Bushmaster mags. Typically the magazines offer seven to 10 rounds in roughly the same space as 20 to 30 5.56, respectively.

Recoil comes only in the form of a slight snap and a pop. It’s more than a 5.56 but less than a 12-gauge in my experience. In fact, I’d say a 20-gauge with buckshot hits the shoulder harder than an AR in .450 Bushmaster. As you get to lighter rifles and bolt guns, the recoil jumps up a peg but is never a shoulder slapper.


The Guns

Speaking of guns, the most popular platforms are 16- to 20-inch barreled AR 15s. However, the .450 Bushmaster might be the most popular large-bore caliber for AR 15s and beyond. The round has found itself in the popular and affordable Ruger American Ranch rifles. Here it comes with a mighty nice compensator to help with recoil.

Single-shot rifles like the Henry and CVA series chamber the thumper. This creates very light and handy rifles that pack a punch. If you want to go baller with a single shot, the Ruger No. 1 comes in .450 Bushmaster as well.

Bolt guns are also an option.

A few companies even make .450 Bushmaster AR pistols but be warned once you get below 12 inches of barrel, your velocity drops off decently fast. You’ll need all the velocity you can get out of this slugger of a round.

The .450 Bushmaster certainly meets Colonel Cooper’s requirements for a Thumper cartridge, and in the AR 15, it’s a perfect Thumper platform. It’s great for brush hunting, hog killing, and dealing with angry bears and happy elk. It’s nice to see the Thumper maintain a presence with the American Sportsman.