Big Horn Armory AR500

A Preposterously Powerful Pistol

They don’t get any bigger! The Bighorn Armory AR500 fires the biggest cartridge
of any AR-platform gun, the 500 Auto Max. Big medicine in a small package!
Photo: Big Horn Armory. An AR-10 lower is broached to accommodate the AR-15
sized magazines (right). The flared magazine well assists in loading magazines.

The voice on the phone sounded familiar. “Hey, Tank! I’ve got an assignment that, ah, no one else wants … er, I mean, an assignment right up your alley!” It was GUNS Editor Brent Wheat.

I take the bait; his presentation perfectly cast like a black hula popper to a lurking large-mouth bass. And I was hungry — who wouldn’t want to shoot an AR-style pistol capable of shooting a bullet with 4,000 ft/lbs. of smack-down energy? The gun we’re talking about is Big Horn Armory’s (BHA) AR500 chambered for the most powerful cartridge in existence for AR-platform guns — the 500 Auto Max!

Big-Bore, Because …

Anything starting with the number five, caliber-wise, is big, right? It’s unnecessary for a large-bore bullet to rely on expansion for performance, especially when it starts out at ½”. As Sir Samuel Baker stated a few centuries ago, a bullet’s weight and diameter are constant while velocity is a diminishing variable.

From Lever to Semi-Auto Rifles

BHA was founded in 2008 by Greg Buchel, while pursuing what was to be named the Model 89 lever gun in .500 S&W, their flagship firearm. In 2017, Big Horn expanded their line with the AR500 Auto Max, the most powerful short-range, semi-auto rifle based on the AR10 platform. Capable of driving 400-grain bullets in excess of 2,000 fps, the AR500 is able to stop anything from pachyderms to Peterbilts — and we know Peterbilts are always in season, especially on the Jersey Turnpike during rainstorms!

Released in 2018, it wasn’t long before customers started requesting the AR500 rifle in a more convenient pistol-size platform, which brings us up to BHA’s current release. The pistol version lacks nothing but length from the rifle, making it handier to wield in tight confines.

For outdoorsmen, this means hunting blinds, tree stands, or stalking in thick cover. Law enforcement/security/military personnel can add this tool as a powerful breeching weapon, able to clear ballistic glass, automobile doors, or other tough barriers as well as handling dangerous animal complaints. What better bear-stopper than a 500 Auto Max pistol? I can’t think of any.

They don’t get any bigger! The Bighorn Armory AR500 fires the biggest cartridge
of any AR-platform gun, the 500 Auto Max. Big medicine in a small package!
Photo: Big Horn Armory. An AR-10 lower is broached to accommodate the AR-15
sized magazines (right). The flared magazine well assists in loading magazines.

AR-15 single-stack steel magazines are used for reliable feeding
of the large 500 Auto Max cartridge.

AR Pistol Platform

To get the real nitty-gritty on the AR500, I spoke directly to Greg Buchel. He told me, “the .500 S&W has a rated pressure of up to 60,000 psi, so to contain this level of pressure, and the accompanying bolt thrust of a case this size, we clearly cannot wrap an AR-15 size receiver around all this ballistic energy.

“We knew we had to start with the AR-10 and use a DPMS-pattern, with the end result being a gun we call the AR500. We also knew we’d have to alter the bolt from .308 size to accommodate the larger case head of the .500 Auto Max from 0.480″ to 0.520″. The .500 Auto Max is a rebated rim, the rim being 0.008″ smaller than the case,” Buchel stated.

BHA engineers positioned the extractor and ejector in ideal locations for reliable function within the bolt face, while also adjusting the extractor to mate perfectly with the rim of the cartridge.

The 10″ 416 stainless-steel Green Mountain barrel has a bullet stabilizing 1:24″ twist, the most efficient length for the 500 Auto Max. It’s given a rugged, surface-hardening and corrosion-resistant nitride bath for a durable finish. The muzzle is threaded 3/4X28 for the accruement of your choice but comes with a muzzle brake installed.

The adjustable gas block allows fine tuning of the gas flow for reliable cycling for bullet weights from 300-grain loads to 600-grain sub-sonic loads and everything in between.

The upper receiver has a full-length rail, extending the length of the handguard. The ejector deflector provides ample clearance for the large, heavy brass of the 500 Auto Max as the ejector tosses the brass with authority 6 to 7 feet at 3 o’clock.

The handguard is a free-float, M-Lok pattern, with a full-length Picatinny rail. At the end of the handguard are integral sections of the rail at the 3, 6 and 9 o’clock positions for any accessories such as light, laser, etc. you may want.

The lower receiver has ambidextrous selector and bolt catch, but no ambidextrous magazine release. Magazines for the AR500 are AR-15 sized, despite the upper and lower being AR-10 size. This is due to feeding geometry and characteristics being better from the AR-15-sized magazines. The steel magazines come in both 5- and 9-round, single-stack configurations.

The AR-10 lower has an AR-15 magazine well broached into it and a standard AR-10 buffer with a proprietary action spring. This mix/match of parts may sound strange but when put together, it works, cycling these powerfully behemoth rounds without a hitch.

Lastly, the BCG is nickel-boron coated to reduce friction and protect it from the elements, while making it easier to clean.

The lineup of 500 Auto Max Ammo is impressive! Left to right: Two rounds
from Steinel, three from Buffalo Bore and last two from Underwood.

Caliber Conversion

The 500 Auto Max is the creation of BHA’s Greg Buchel and Tim Sundles, head honcho of Buffalo Bore Ammunition. The 500 Auto Max is simply the .500 S&W with the rim turned down to basically rimless status so it can function reliably through the AR platform. In fact, the .500 Auto Max can chamber and fire in any revolver or single shot chambered for the 500 S&W.

For handloaders, the same reloading data is used, as well as the same dies and shell holder, for the .500 S&W. However, the 500 Auto Max requires a taper crimp as the cartridge headspaces off the case mouth. Ammunition is available and currently shipping from Buffalo Bore, Steinel and Underwood Ammo.

Compared to the .50 Beowulf, the 500 Auto Max pushes same weight bullets 2,300 fps faster incomparable results.

The bolt-face was opened up to accept the 500 Auto Max head.
Extractor and ejector were strategically placed for optimal performance.

The 500 Auto Max dwarfs the .45 Colt and .480 Ruger cartridges.
It’s a beast and that’s why Tank loves it!

The 500 Auto Max (above right) is actually just a .500 S&W cartridge
with the rim turned down for use in a semi-auto.

Why A .500 Auto Max?

When asking Buchel who his best customers are, he responded, “just regular firearm enthusiasts who enjoy shooting the biggest and baddest guns around!” And he’s right! Shooting the AR500 Auto Max is exhilarating! Watching the rifle berm explode in a shower of dirt clods as the huge bullets strike is a firsthand lesson in energy transfer and momentum.

Better yet is shooting items such as cinder blocks, old propane tanks or abandoned automobiles to conduct your own ballistics tests. Need to demo a brick wall? Why not do it the fun way? The 500 Auto Max is up for the job. Shooting this beast from the hip is also exhilarating, pressing the trigger as fast as you can, sending huge hunks of lead downrange and watching them destroy whatever is in your shooting path.

Typical accuracy from the AR500 Auto Max pistol at 100 yards. Groups
ranged from 0.9 to 2" as these groups demonstrate. Great accuracy for
such a powerful pistol.

At The Range

I added a Rock River Arms Hi-Rise One Piece Cantilever mount and an old, reliable Weaver 2-10X40 scope to achieve utmost accuracy. The trigger of the AR500 Auto Max was crisp and creep-free, breaking at just over a measured 3 lbs.

After taking two shots to zero the scope, I placed my target stand at 100 yards and commenced firing 3-shot groups with several different loads from Buffalo Bore, Steinel and Underwood.

Everything shot within 2″, with most running around 1.5″ for 3-shot groups at 100 yards. A few groups were 1″ or less. This is extraordinary accuracy, especially from such a short, powerful pistol!

The Final Word

The BHA 500 Auto Max pistol is a great way for people to enjoy a compact gun without the taxing paperwork involved of owning an SBR — being classified as a semi-auto pistol, with fixed-blade stabilizer bar, has its merits.

For handloaders, loading 500 Auto Max is as easy as following load data for the .500 S&W, only using a tapper crimp die, instead of a roll-crimp die.

Lastly, shooting the AR500 Auto Max is fun! Americans like things big, and right now the AR500 chambered in the 500 Auto Max is as big as it gets for the AR Platform. So, go out and have some fun — knock down a cinderblock wall, shot by shot, or shoot a bear, bison, or Buick. I guarantee, whatever you shoot, it’s not going anywhere but down!
MSRP: $2,199

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