big horn armory ar500 rifle
Bryce M. Towsley

When Eugene Stoner was asked to scale down his AR-10 design to fit the much smaller 5.56 NATO cartridge, he never could have imagined the journey that rifle would eventually take. That basic design has gone from the glitch-filled early M-16 rifle that was despised by the soldiers fighting in Viet Nam who were forced to use it, to the most popular sporting rifle in America today. The M-16 rifle was soon was entrenched with the military, but for the civilian world, the AR-15 design was a late bloomer. In the new millennium, though, it has grown into its potential as a sporting rifle. The AR-10 design has tagged along on its coattails and both have become popular with hunters. Both of these rifle designs limit cartridge length, which also limits potential power. The AR-10 uses the 2.8-inch, .308 Winchester family of cartridges which are fine for medium size big game. The AR-15 limits the length of the cartridges even more to the 2.260-inch length of the .223 Remington. To progress to big game hunting status, cartridge designers have made the bullets bigger, spawning cartridges like the .450 Bushmaster, .458 Socom, and .50 Beowulf. All are big bore cartridges using heavy bullets at moderate velocity. Scaling Up and Down Greg Buchel, the owner of Big Horn Armory, has been making lever action rifles chambered for the .500 S&W handgun cartridge. In the longer barrel of a rifle this cartridge edges into the realm of beast mode. He makes it work in the older lever action designs by using modern materials and manufacturing processes that increase the strength and reliability of the firearms.

big horn armory AR500
Big Horn Armory AR500 rifle in .500 Auto Max. Bryce M. Towsley

Buchel watched the growing AR market and thought, “why not put this power into an AR rifle?” The resulting rifle is a bit of a hybrid of the AR-15 and AR-10 designs. It’s called the AR500 and the redesigned cartridge is called the .500 Auto Max.


The cartridge is capable of driving a 350 grain bullet to 2,300 ft/s and a 400 grain bullet to 2,050 fps. The 440 grain has a M.V. of 1,900 ft/s. This puts the muzzle energy of this cartridge on one side or the other of 4,000 foot-pounds, depending on the bullet. That is in the same category as the famed .375 H&H, which has taken all the big game in the world.

That means that the .500 AM cartridge is very capable of handling anything in North America. It can also handle dangerous game like Cape buffalo, except that using an AR style rifle in Africa is very difficult, if not impossible, due to the laws restricting firearms. But if you want an AR rifle to hunt deer, hogs and black bear with, one that is capable of taking Alaskan brown bear if your dream comes true, this is your huckleberry.

Even though the overall length of the .500 Auto Max cartridge will fit in an AR-15, this is a very powerful cartridge running at extremely high pressure. The smaller AR-15 was not strong enough to safely use with this cartridge, so Big Horn Armory redesigned the larger AR-10 to accept the smaller AR-15 magazines.

The .500 Smith & Wesson is designed for a revolver, and as such it has a rim on the case. Rims have always been problematic when trying to feed cartridges stacked on top of each other in a magazine.

To create the .500 Auto Max, Big Horn Armory worked in conjunction with Buffalo Bore ammo to remove the rim and make the new cartridge rimless. No other significant changes were made and the two use the same loading data. While the .500 S&W is very powerful, the longer barrel adds to the velocity and the .500 Auto Max is even more powerful. The muzzle energy of the same bullet used in the handgun is often doubled when fired from the rifle.

This is a short range cartridge by today’s standards, but so what? The vast majority of big game is shot at distances well under 200 yards and this cartridge can handle that distance easily. For example, the ballistics tables for the 400 grain load show that it maintains 1,500 foot-pounds of energy at that distance. Enough for deer or even moose.

The Big Horn Amory AR500 rifle weighs 9 pounds, 6 ounces. It has AR-10 size machined billet receivers made from 7075-T6 aluminum alloy. The receivers do not interchange with any other AR-10 rifle as they have unique features designed around this cartridge. The 18-inch barrel is button rifled with a 1:24 twist rate.

The barrel bore is treated with a salt bath nitride finish. This makes the barrel last longer and easier to clean.

big horn armory ar500 muzzle brake
The muzzle brake on the Big Horn Armory AR500 rifle in .500 Auto Max. Bryce M. Towsley

The rifle is threaded at the muzzle and the gun comes fitted with a recoil reducing brake.

The gun is also fitted with an adjustable gas block.

A Word on Resistance

A word on that. Buchel pointed out that the gun needs significant resistance from the shooter’s shoulder to function properly, meaning it won’t cycle if there isn’t enough meat behind it and it moved back and forth too much during firing.

It runs well with most adults using conventional shooting positions. I am 240 pounds and my shooting buddy, Mike Brookman who helped with this test is the size of an average Wookie, so we didn’t encounter any problems, except when shooting from a bench rest where I had a few glitches in feeding.

I will note that function issues when shooting from a bench rest are common with AR type rifles, so I don’t worry about them much. The gun functioned fine when shooting it from any other position. If there are problems, say a shooter is too small to provide enough resistance, the adjustable gas block should solve them.

Other Features

The front handguard has a full length Picatinny rail. It accepts M-Lok attachments to add accessories like a laser or flashlight.

big horn armory ar500 magazine
The Big Horn Armory AR500 rifle in .500 Auto Max is on an AR-10 size platform, but the magazine well has been reduced in size to fit an AR-15 type magazine. Bryce M. Towsley

The trigger guard is oversized, so hunting and shooting with gloves isn’t a problem. The mag well has a machined-in block to reduce it to take the smaller AR-15 magazines. The top of this section is shaped to a long feed ramp to guide the cartridges from the magazine into the chamber.

The safety and slide release are ambidextrous. The trigger pins are anti-rotational and fitted with a locking screw. The single stage trigger breaks cleanly with no hitches or drag at 3-3/4 pounds, which is just about perfect in an AR style rifle.

The gun is fitted with a standard, rubber Ergo grip, which for the record is my preferred grip on an AR rifle. The buttstock is adjustable for length and is fitted with a soft, thick recoil pad. There are quick release sling fixtures on the buttstock, receiver and on the front rail, on both sides of the gun.

magazine for big horn armory ar500
This magazine for the Big Horn Armory AR500 rifle in .500 Auto Max has been modified and will hold 5 rounds. Note the different follower. Bryce M. Towsley

The rifle ships with a 5-round magazine. It’s basically a 20-round .223 magazine that has been modified for use with this cartridge. Big Horn Armory is working on higher capacity 10 and 15 round magazines and should have them ready soon.

Range Test

We shot the gun with several ammo products from Buffalo Bore and from Underwood Ammo. Like most rifles, this loved some ammo and didn’t play well with others. Both of their 350 grain loads shot very well in terms of accuracy, as did the 400 grain soft nose Buffalo Bore ammo.

various ammo used in rifle test
There is a wide range of factory loaded ammo for the Big Horn Armory AR500 rifle in .500 Auto Max. Buffalo Bore and Underwood Ammunition both offer several bullet options. Bryce M. Towsley

This is currently the most powerful AR specific cartridge. (At least as far as I can tell. There are new cartridges popping up everywhere these days.) For a hunter, it is a hammer on hogs and if they ever get this cloning thing perfected it should be just right for hunting Velociraptors. Remember, they travel in packs, move fast and can be tough. A fast handling rifle with a high capacity magazine and a bludgeon class cartridge is just about perfect.

The cartridge has seen some LE and Military use for breaking parts on bad guys and their vehicles. If you have to stop a mad bomber’s approaching truck, this is the gun for you.

In terms of shooting fun, this thing is way on the right side of the bell curve. Shooting reactive targets like knock down steel, rocks, or water jugs will make even the most sourpuss of shooters giggle like a kid in middle school.

buffalo bore ammo shot group
A grouping of Buffalo Bore .500 AutoMax ammo. Bryce M. Towsley

Shooting Results

All groups are at 100 yards and fired from a bench rest with sandbags. Average of three groups. Velocity is measured with an Oehler 35P Chronograph with the first screen 15 feet from the muzzle.

Buffalo Bore 350 grain JHP

  • Velocity: 2,288 ft/s
  • Average group: 2.7 inches

Buffalo Bore 400 grain FP

  • Velocity: 1,710 ft/s
  • Average group: 2.3 inches

Underwood Ammunition 420 grain Extreme Penetrator

  • Velocity:1,744 ft/s
  • Average group: 2.5 inches
measuring ammo diameter
The half-inch diameter bullets used for the Big Horn Armory AR500 rifle in .500 Auto Max make a big hole. Bryce M. Towsley

Big Horn Armory AR500 Specs

ACTION TYPE: Direct-impingement semi-auto
CALIBER: .500 Auto Max
FINISH: Anodized black on the receivers. Barrel is black oxide.
BARREL LENGTH: 18-inches
RIFLING: 1:24 Button rifling.
SIGHTS: Rail for mounting optics
STOCK: six position adjustable
LENGTH: 37 to 40.25 inches
WEIGHT: 9 pounds 6 ounces
MSRP: $1,999.00

*Other magazines with higher capacity will soon be available.