Big Horn Armory's AR500 AR-Platform Semi-Auto Rifle in 500 Auto Max

by Boge Quinn

March 14th, 2022

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AR500 Auto Max Semi-Automatic Rifle from Big Horn Armory

by Jeff Quinn

photography by Jeff Quinn & Boge Quinn

September 21st, 2018

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It was almost two decades ago that I took an intense interest in big-bore AR rifles; first the 499 Leitner-Wise and then the 50 Beowulf. Both were based upon the idea of firing a fifty-caliber bullet from the AR-15 platform. I settled upon the Beowulf, and have been well-satisfied with it, using it on hogs and black bear with great success. A half-inch diameter bullet weighing between 350 and 400 grains and traveling just under 2000 feet-per-second (fps) works handily on large critters, which has been the success of the Beowulf cartridge and the rifle which fired it. Now, the new 500 Auto Max cartridge and the AR500 rifle takes it a step farther, delivering higher velocities and flatter trajectories from a rifle of similar design.

The AR500 is built on the larger AR-10 platform, but has the magazine well modified to work with a smaller magazine than that which is required for 7.62x51mm/308 Winchester class cartridges. The 500 Auto Max cartridge is basically a 500 Smith & Wesson Magnum with the rim milled off, so that it will feed reliably from a box magazine. The rifle is produced by Big horn Armory in Cody, Wyoming, and the ammunition is produced by Buffalo Bore Ammunition in Salmon, Idaho using Starline cartridge cases. Both companies worked together on this project, producing a fifty-caliber semi-auto cartridge that produces velocities exceeding those of the Beowulf cartridge. The 500 Auto Max beats the Beowulf velocities for any given bullet weight by between 235 and 340 feet-per-second (fps).

I fired the three different Buffalo Bore loads over my chronograph to check velocities, and the results were either pretty close or even exceeded the velocities listed by Buffalo Bore. All velocities were checked twelve feet from the muzzle of the Big Horn rifle, at an elevation of 541 feet above sea level, with an air temperature of ninety-one degrees Fahrenheit with humidity in the ninety percent range. Typical Tennessee Valley Summertime. Velocities are listed in the chart below. Velocities are listed in feet-per-second. Bullet weights are listed in grains. JHP is a jacketed hollowpoint bullet. JFN is a jacketed flat-nose bullet. HCL is a hard-cast flat-nose lead bullet. Also listed are 50 Beowulf velocities checked on the same day under the same conditions. The Beowulf ammunition was fired from an Alexander Arms sixteen-inch barrel.

Ammunition Bullet Weight Velocity

500 Auto Max



Buffalo Bore JHP 350 2260
Buffalo Bore JFN 400 1930
Buffalo Bore HCL 440 1911

50 Beowulf



Alexander Arms 334 1922
Alexander Arms 400 1697

Recoil, while noticeable, was not painful at all. The rifle does back up a bit when the trigger is pressed, but the recoil is straight-back, and doesn’t slap the shooter’s cheek at all. The nearly ten-pound rifle soaks up the recoil very well. Even the 440 grain Buffalo Bore load was easy to shoot. The rifle packs a lot of power, and getting back on target after the shot is quick and easy. Functioning with the factory Buffalo Bore ammunition was flawless. I did experience a couple of failures-to-extract with the bulk ammo I had on hand, but I do not know what components were used in assembling those loads. The AR500 doesn’t really recoil any harder than does the 50 Beowulf; even though the 500 AR has more power than the Beowulf, the gun weighs about two and one-half pounds more, which soaks up the extra recoil.

The eighteen inch barrel measures .865 inch diameter just forward of the handguard, and is threaded and fitted with a muzzle brake. The barrel is rifled one turn in twenty-four inches. The twelve and five-eighths-inch length handguard is aluminum, and wears a full-length Picatinny-spec rail its entire length on top, with three other small rail sections screwed on at the 3, 6, and 9 O'clock positions. These small rail sections can be moved to various places on the handguard, if desired. The metal is finished overall in a matte black. The AR500 rifle wears ambidextrous safety levers and bolt catches. The magazine catch is of the high-profile extended type. The rifle ships with one five-shot magazine. The six-position adjustable buttstock is better than most of its type; easy to adjust, and comfortable to use. Most telescoping buttstocks pull the hair out of my face, but I had no trouble at all with this one.

Accuracy was excellent! The big 500 would cluster five shots into one ragged hole at fifty yards, and would do almost as well at one hundred yards, shooting from the bench using a Target Shooting, Inc. Model 500 Rifle Rest. I like the Model 500 for ARs, as the extended magazines cause no problems or interference. For all accuracy testing, I mounted a Leupold HOG low-magnification variable scope. This is an excellent scope sight for such a rifle, and can easily withstand the recoil from this AR500.

The Big Horn rifle is a reliable and accurate AR-based weapon for hunting large game whether up close and personal, where a fast succession of shots might make the difference between a good hunt and a really bad day, or out to extended distances. The Big Horn AR500 can handle it. The big AR500 ammo packs enough power to handle any game on Earth, and with a magazine full of cartridges available as fast as one can press the trigger, the Big Horn rifle can deliver that power on target quickly.

Check out this and other Big Horn products online at

For top-quality ammunition, go to

Jeff Quinn

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Click pictures for a larger version.







Cartridge Comparison: 50 Beowulf (left), 500 Auto Max (right).







Buffalo Bore's loads for the 500 Auto Max (left to right): 350-grain JHP @ 2260 FPS, 400-grain JFN @ 1930 FPS, 440-grain HCL @ 1911 FPS.