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Rock River Arms LAR-15M .450 Bushmaster - Reviewed & Tested

The .450 Bushmaster rifle cartridge is seeing a surge in popularity with new factory ammo and AR carbines like the Rock River Arms LAR-15M; here's a full review and test report on how the rifle performed with the "thumper" cartridge.

Rock River Arms LAR-15M .450 Bushmaster - Reviewed & Tested
Photos by Michael Anschuetz

The concept for the .450 Bushmaster cartridge originated with the late Col. Jeff Cooper (1920–2006) of Gunsite fame. Cooper was less than enamored with the .223 Remington cartridge, and in a book published in 1998, he envisioned a more powerful, large-caliber round that would reliably take down big-game animals. Significantly, the round should be adaptable to the AR-15/M16 platform. Cooper called his concept cartridge “Thumper.”

Just shy of a decade later, what eventually became the .450 Bushmaster cartridge was designed by Tim LeGendre of LeMag Firearms and licensed to Bushmaster Firearms International. The case is based on the .284 Winchester, which has a fat, 0.500-inch base and a rebated rim that is the same diameter as the .30-06 Springfield for boltface compatibility. The original .450 Bushmaster case length was 1.772 inches, and the overall cartridge length (COL) was set at 2.362 inches. This made it a bit too long for the AR.

Sometime around 2007, Hornady was asked to manufacture the ammunition, but the company wanted to use its 0.452-inch Flex Tip eXpanding (FTX) bullet for improved downrange ballistics and to fit in ARs. The 0.452-inch bullet was selected because the velocities produced by the Bushmaster’s relatively small case wouldn’t be high enough to force tougher 0.458-inch bullets to expand. Hornady shortened the case to 1.700 inches, made the COL 2.26 inches to fit in the AR, and set the maximum average pressure (MAP) at 38,500 psi, well below the 55,000 psi MAP of the .223 Rem. The .450 Bushmaster was accepted by SAAMI, and Cooper’s “Thumper” became a reality.

Rock River Arms LAR-15M
Col. Jeff Cooper came up with the .450 Bushmaster concept, and today plenty of factory-loaded ammunition is available. There are even a few new-for-2020 offerings. (L-R) Hornady Subsonic 395-Gr. Sub-X, Hornady American Whitetail 245-Gr. SP, Hornady Custom 250-Gr. FTX, Remington Premier 260-Gr. AccuTip, Federal Non Typical Whitetail 300-Gr. HP, Federal Fusion 260-Gr. BSP, Winchester Deer Season 250-Gr. Extreme Point

The .450 Bushmaster is moderately popular, but for reasons that are profoundly disparate. First, the round can stand on its merits; it’s accurate, powerful, versatile, and adaptable to ARs as well as bolt-action rifles. Second, there are the convoluted criteria of the various state Fish and Game agencies. One of these is to arbitrarily mandate the parameters of cartridges that can be used for hunting game. The regulations of several states specify case and/or cartridge length, straight case walls, bullet diameter, and other dimensional minutiae. (You will recall that such reasoning influenced the development of the more recent .350 Legend.) That said, today a good selection of .450 Bushmaster ammo is available from Federal, Remington, Winchester, and Hornady. One of Hornady’s new-for-2020 factory loads is the 395-grain Sub-X loading in the company’s Subsonic ammunition line. And, of course, a lot of good handloading components for the .450 Bushmaster are also available.

The Carbine

To climb on the Bushmaster bandwagon, I ordered a brand-new Rock River Arms (RRA) LAR-15M. It is top drawer. The stainless-steel cryo-treated barrel is 16 inches long, has a 1:24-inch twist, and comes with an RRA Operator muzzle brake installed. Checking the carbine’s bore with my Hawkeye borescope showed the rifling has no scratches or extraneous tool marks. The LAR-15M has an efficient low-profile gas block and a carbine-length gas system.

The buttstock is RRA’s CAR six-position stock that’s adjustable for length of pull (from 11.25 to 15 inches). There are no sights, but there is a full-length Picatinny rail on the receiver and on the 13-inch-long, free-floated handguard. With the Trijicon scope used for testing, the rig weighed 9 pounds, 11 ounces. The RRA two-stage trigger broke smoothly and crisply at a delightful 4 pounds, 8.3 ounces.

Rock River Arms LAR-15M
The LAR-15M comes with Rock River’s excellent two-stage trigger. Steve found it to be crisp, smooth, and consistent, with an average trigger pull of 4.52 pounds.

The gun’s forged lower is mated to an A4 upper, and all parts are the high-end components that most AR users eventually put on their rifles, i.e., flat-top Picatinny rail, lightweight free-floated handguard, overmolded pistol grip, and adjustable buttstock.

Range Performance and Results

I gave the LAR-15M a workout on the range, and the slick carbine shot pretty darn well with the variety of factory ammo and handloads I put together for this report. Due to the popularity of the cartridge, there are at least nine factory loads with bullet weights that range from 245 to 395 grains, and I was able to shoot seven of them: three from Hornady, two from Federal, and one each from Remington and Winchester. The overall group average of these seven loads in the LAR-15M was 1.29 inches at 100 yards.

The .450 Bushmaster is obviously intended for hunting deer, hogs, and black bears, and the available loads reflect that. Out to about 150 yards or so, they should be devastating. Plus, they have very manageable trajectories. Hornady’s American Whitetail load features a 245-grain softpoint bullet rated at 2,200 fps. It clocked 1,928 fps out of the LAR-15M and averaged a tidy 1.10 inches. The company’s Custom load carries Hornady’s 250-grain FTX bullet with a sharp ogive and a polymer tip that ups the bullet’s ballistic coefficient and also helps initiate expansion on game. It is listed at 2,200 fps and averaged 1,997 fps and 1.30 inches in my tests.

The downrange ballistics of the 250-grain FTX load are impressive, if a bit surprising. When sighted-in one inch high at 100 yards, this bullet is 4.01 inches below the line of sight at 175 yards and 7.68 inches down at 200 yards, where it still delivers 1,045 ft-lbs of energy.

Remington offers a 260-grain AccuTip in its Premier line, and it registered 1,968 fps and averaged 0.91 inch in the LAR-15M. Winchester’s Deer Season ammo has a 250-grain Extreme Point bullet that averaged 1.10 inches at a velocity of 2,077 fps. Federal’s Fusion ammo carries a 260-grain bonded softpoint bullet rated at 2,200 fps, and it measured 1,947 fps with an average accuracy of 1.55 inches. Federal’s Non Typical White Tail load shoots a 300-grain hollowpoint bullet, and it averaged 1,793 fps and 1.61 inches.

Rock River Arms LAR-15M
The LAR-15M achieved 100-yard five-shot group averages ranging from 0.77 inch to 1.61 inches with factory-loaded ammo and handloads.

Shooting the .450 Bushmaster is an adventure. It’s accurate, all right, but you know when you’ve touched it off! It kicks (for recoil figures, my handloads ranged from 9.5 ft-lbs to 14.6 ft-lbs), bellows, and emits huge bright orange balls from the muzzle, but anything shot with it is apt to stay shot.


I never base my shooting of a cartridge or rifle on factory fodder alone, and the .450 Bushmaster is literally a blast to handload. The Bushmaster case headspaces on the mouth, so as we learned with the straight-wall .350 Legend rifle round, .450 Bushmaster cases need to be the exact same length for consistent ignition and accuracy. For this report, I used Hornady and Starline brass trimmed to 1.690 inches (0.010 inch shorter than the SAAMI maximum case length).

A second proviso deals with crimp, which also affects headspace. As Hornady recommends, it is important to not use a heavy roll crimp with bullets that have cannelures because this can cause dangerous conditions. Hornady die sets for the .450 Bushmaster come with a separate taper crimp die. All of my handloads were gently taper crimped to a muzzle diameter of 0.387 inch, the same as most factory loads, and all functioned perfectly.

Only a few powders are suitable for the .450, and it doesn’t take many handloads to learn that Hodgdon’s Lil’ Gun is far and away the best choice. It produced the highest velocity with all bullets tested and good accuracy with all bullets. Top velocities were about 2,000 fps with all loads. But H110, W296, and IMR 4227 also performed well, so they’re certainly worth trying. An important finding was that IMR 4227 and IMR 4198, both extruded powders, produced noticeably less muzzle flash as compared to the spherical powders. Accuracy was good, although velocities were somewhat lower.

The only hiccup was with chronographing. The .450 Bushmaster’s muzzle blast caused numerous chronograph errors, with apparent 100- to 200-fps swings in velocities. So I built a baffle with a 2.75-inch hole and placed it 6 inches in front of the first chronograph skyscreen. Problem solved.

Rock River Arms LAR-15M
The .450 Bushmaster’s muzzle blast was significant and caused chronograph errors. Steve solved the problem by building and installing a baffle in front of his chronograph.

Now, about the muzzle brake. It has four large slots on each side, angled back at about 45 degrees. It directs a gigantic ball of hot gas, not to mention some unburned powder particles, rearward toward the shooter, so I strongly recommend extra-dark shooting glasses to dim the blinding blaze-orange muzzle flash. Fortunately, the muzzle brake is easy to remove, so I removed it for much of my shooting. Note that RRA does not offer a 5/8-32-thread protector, but Down Range Products in Lenexa, Kansas, does. I also strongly urge shooters to never shoot this gun without ear protection.

Rock River Arms LAR-15M
According to the author, the muzzle brake was obnoxiously loud, so he did most of the shooting with it removed from the barrel.

The LAR-15M in .450 Bushmaster is nicely made, well finished, and has a very good trigger. It gave the results one would expect for a big-bore, medium-range cartridge. Accuracy was acceptable, and most critters would quickly lose an argument with its power. Chambered for Colonel Cooper’s “Thumper” cartridge, the LAR-15M is a potent package.

Rock River Arms LAR-15M
*This low-velocity load did not cycle the carbine’s action. NOTES: Accuracy is the average of three, five-shot groups fired from a benchrest. Velocity is the average of five rounds measured 10 feet from the gun’s muzzle. Range temperature was 45 to 64 degrees Fahrenheit. All load data should be used with caution. Always start with reduced loads first and make sure they are safe in each of your guns before proceeding to the high test loads listed. Since Shooting Times has no control over your choice of components, guns, or actual loadings, neither Shooting Times nor the various firearms and components manufacturers assume any responsibility for the use of this data.

Rock River Arms LAR-15M Specs

  • Type: Direct-impingement gas-operated autoloader
  • Caliber: .450 Bushmaster
  • Magazine Capacity: 5 rounds
  • Barrel: 16 in.
  • Overall Length: 33.75 to 37.5 in.
  • Weight, Empty: 7.3 lbs.
  • Stock: RRA Operator CAR and A2 pistol grip
  • Length of Pull: 11.25 to 15 in.
  • Finish: Hard-anodized black
  • Sights: None
  • Trigger: 4.52-lb. pull (as tested)
  • MSRP: $1,100
  • Manufacturer: Rock River Arms

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