I don’t often get the chance to review a record-breaking firearm. The usual suspects in my T&E lineup – bolt action .308’s, semi-auto CCW’s, and pump-action shotguns – are functional and fun to shoot, but few are as unique as the AR500 from Big Horn Armory.

The AR500 is, according to the company’s claims and my own research, the most powerful short-range semi-auto rifle in the world. Chambered in the .50-caliber 500 Auto Max cartridge, the AR500 packs a wallop in a modified AR-10 platform. Its semi-auto action is perfect for big and dangerous game hunters, and it offers the maneuverability and customizability inherent in the AR platform. Combined with Buffalo Bore’s 350, 400, and 440 grain loads, the AR500 is ready to bring down, quite literally, any kind of critter you’re hoping to hunt.

First, spec sheet/feature list.

• 1:24 twist
• 18″ Barrel
• Adjustable gas block
• Full Picatinny rail
• M-Lok handguard
• Adjustable Buttstock
• Muzzle brake
• Thread Pitch: 3/4×28
• Carbine Length Gas Tube
• Weight: 9lbs, 1.7oz unloaded
• 5-round magazine
• Hard plastic case
• MSRP $1999.00

500 Auto Max?

You’ve never heard of this cartridge because it didn’t exist before Big Horn Armory began developing the AR500. As the folks at Buffalo Bore Ammunition explain, Greg Buchel, the owner of Big Horn Armory, approached the ammo company for help developing a .50-caliber cartridge suitable for the AR-10 platform.

The result was the 500 Auto Max, a reduced-rim 500 S&W currently available loaded with three projectiles: a 350 grain jacketed hollow point, a 400 grain jacketed flat nose, and a 440 grain hard cast. Buffalo Bore plans to expand their offerings in the coming months using everything from 600-grain projectiles to light, fast JHP rounds. The company was kind enough to send me samples of each load currently in production, and they sell boxes of 20 on their website for $67.00 ($3.35/round).

If that sounds a little steep, keep in mind that once you accumulate a collection of AR500 brass, you can reload this ammunition for a much lower cost. According to Buffalo Bore, 500 S&W load data can be used to make this cartridge, though they warn that “care must be given to powder selection to keep from gumming up the AR action.” You can also chamber and fire AR500 ammo in any single-shot or revolver chambered in 500 S&W (though the reverse is not true – don’t try shooting 500 S&W in your AR500). One caveat: Buffalo Bore warns that because AR500 cartridges headspace on the case mouth, they lack the roll crimp necessary to stop the bullet from jumping forward when subjected to heavy recoil. So, if you use AR500 rounds in a revolver, it might be wise to shoot them one at a time.

However you choose to use the cartridge, you’ll smack whatever happens to be downrange. In my testing, the 350-grain projectile average 2207 fps at the muzzle, the 400-grain averaged 1960 fps, and the 440-grain averaged 1838 fps. To provide some perspective, I compiled a chart listing a variety of common cartridges chambered in the AR platform. I’ve included both kinetic energy (foot-pounds) and John Taylor’s “Taylor Knock Out” number. Unlike the kinetic energy formula, the Taylor KO formula accounts for bullet diameter and is generally considered a better way to compare the effectiveness of different rounds against big game at certain velocities (the higher the number, the better).

Cartridge	Bullet Weight (g)	Muzzle Velocity (FPS)	Foot-Pounds of Energy	Taylor KO
.223/.556	62	2900	1158	5
300 BLK	125	2200	1344	11
.308 WIN	168	2600	2522	18
300 WM	185	2900	3456	22
450 Bushmaster	250	2200	2687	35
50 Beowulf	350	1820	2575	45
AR500	350	2207	3786	55
AR500	400	1960	3413	56
AR500	440	1838	3301	57

 

None of these numbers account for bullet construction, and a 300 WM will be more effective than a 450 Bushmaster, for example, at longer distances. Still, it’s clear from both kinetic energy and TKO that the AR500 makes a convincing argument as the most powerful semi-auto short-range rifle on the market.

The 500 Auto Max’s maximum effective range will depend upon the target, the shooting conditions, and, of course, the shooter. I won’t pretend to be a terminal ballistics expert, but my suspicion is that this cartridge can push a projectile at lethal velocities far beyond its accuracy potential. Based on the results of my testing (below), you can confidently put rounds in the kill zone out to about 400 yards, depending, again, on the target.

Buffalo Bore has also published complete data charts on each load (350g, 400g, 440g), which provide tons of helpful information for a variety of muzzle velocities.

Features

But enough about the cartridge. The AR500 feels, looks, and shoots (more on the below) like a $2,000 rifle. Big Horn Armory packed their record-setting long gun with tons of great features, all of which provide either aesthetic or practical (but mostly practical) value. The great-looking Ascend Armory receivers are constructed from 7075-T6 billet aluminum and feature enhanced AR-10 front and rear take down pins, ambi bolt catch/release levers, anti-rotation trigger and hammer pins, and an oversized magwell and trigger guard. The safety selector is also ambidextrous, but the mag release is right hand only. The pistol grip is from ERGO Grip and is totally rubberized.

I think the receivers are sharp, but more importantly, they fit together perfectly. The receivers on the model I tested didn’t exhibit any noticeable movement while still maintaining ease of disassembly.

The massive bolt and bolt carrier group are nickel-boron coated for added reliability. Otherwise, they’ll look familiar to anyone already accustomed to the AR platform. The trigger is excellent—just what you want for a once-in-a-lifetime big-game hunt. It’s crisp, consistent, and relatively light. On the model I tested, the trigger broke cleanly at 4 pounds, 8 ounces.

Moving to the rear of the gun, the Adaptive Tactical buttstock is adjustable and features a healthy recoil pad. The stock works fine, though I found it wobbles quite a bit. The wobble didn’t affect performance, but stock wobble is one of my pet peeves, and I’d wager I’m not alone. The castle nut can be staked to keep it from spinning loose.

Moving forward, the handguard extends nearly to the end of the barrel and features M-Lok cutouts, a complete top Picatinny rail, and short rail sections on the front and rear at the 3, 6, and 9-o’clock positions. It also includes quick-detach sling mounts on both the right and left-hand sides, which pair with the QD mounts milled into the receiver. Finally, the 18” 416 SS barrel is coated with nitride and topped with a much-needed eight-hole muzzle brake.

Reliability, Accuracy, and Performance

The AR500 is a reliable rifle, provided the shooter can maintain a firm grip. Much like the 1911, this beast requires a strong hand, and I found that shooting from the prone delivered better results than from the standing or kneeling positions. Greg Buchel, the owner of Big Horn Armory, confirmed my suspicions. He explained that it’s important for the shooter to provide a solid base against which the rifle can recoil. They’ve found that strong shooters using proper technique can run an AR500 without any issues, while other shooters can have trouble using the same firearm.

He also noted that their rifles generally need a 100-200-round break-in period before they’re running at peak performance. If the rifle is still malfunctioning after 200 rounds, the adjustable gas block can be tuned down from fully open to ½ or 1/3, which usually makes the gun run more smoothly.

I experienced three failures to extract during the course of my testing, but none while shooting from the prone position. I guess I need to work on my form—or hit the gym. If I planned to keep the rifle long-term, I’d also consider adjusting the gas block.

In terms of accuracy, Buchel said they expect 1.5 MOA groups from the AR500. The groups I shot tended to be larger, though for a rifle of this caliber I’d be happy with anything under 2 inches. I shot these groups from 100 yards using a Caldwell Lead Sled. The wind was minimal, and the temperature was right around 96 degrees.

Bullet	Average Velocity (FPS)	Smallest Group (in)	Average Group (in)
350g JHP	2207	1.8	2.3
400g JFN	1960	1.8	2.2
440g HC	1838	3.5	5.2

 

The 350g and the 400g loads performed well, but my rifle didn’t favor the 440g hard cast bullets. I thought perhaps the barrel was too hot or my scope had lost its zero, so I shot another group with the 400g load immediately after the first 440g group. The 400g group shrank down to normal size, so I ruled out any abnormal rifle or scope malfunctions.

Buchel told me they haven’t tested the rifle with the 440g hard cast load, so they aren’t sure what kind of accuracy to expect. It’ll do its job on a charging bear from 50 yards, but I’d need to conduct more testing before going after game with the 440g.

The rifle handles well. I found it to be somewhat front-heavy, but it’s light enough to land shots in the standing position. Recoil feels much stiffer than a standard AR-10, but the muzzle brake and recoil pad keep it to a manageable level (something akin to a 45-70). The muzzle brake allows for relatively quick follow-up shots, but don’t expect your shot splits to rival the performance from an .308 AR-10.

Central Texas is decidedly lacking in elk, moose, or grizzly bear, so I can’t speak directly to the rifle’s performance against large and dangerous game. But I have found the water jug test to be a decent yardstick for bullet behavior, so I decided to give it a try.

• The 350g JHP blasted through two one-gallon water jugs, fragmented, and penetrated a third.
• The 400g JFN blasted through five one-gallon water jugs, opened, and penetrated a sixth.
• The 440g HC blasted through nine water jugs and lodged in the stone backstop

The results of the test are consistent with what Buffalo Bore says to expect from each load: 1) the 350g is designed for game up to 600 pounds and will likely disintegrate on big shoulder bones at close range, 2) the 400g is designed for game up to 1500 pounds and will over-penetrate deer-sized animals, and 3) the 440g is designed for animals up to 3000 pounds and will penetrate deeply and straight.

For perspective, I conducted the same test last year using .556, .308 WIN, and 450 Bushmaster. The .556 penetrated two jugs, the .308 penetrated five, and the 450 Bushmaster penetrated four. While water jugs aren’t a perfect stand-in for animal flesh, the test confirmed the expected bullet behavior and suggests how the 500 Auto Max might compare with other AR cartridges in terms of penetration power at close range.

Conclusion

For me, the most exciting benefit of the AR500 is its combination of cartridge and platform. The 500 S&W has been around since 2003, and the AR platform has been in production since the 1950s. Combining the two gives users the power of the 500 S&W with the rapid fire/reload functionality of the AR-10. For decades, anglers have used big-bore revolvers and lever action rifles for bear defense in harsh northern terrain, but the AR500 provides better accuracy and handling than a revolver and faster shot splits and reloading than a lever action. If I’m staring down an angry bear at 50 yards, I want the AR500.

The same holds true for large and dangerous game. If I’m on the hunt of a lifetime in Africa or Canada, fast follow-up shots can ensure the critter won’t get away, and I’ll have a trophy for my efforts (not to mention hundreds of pounds of meat).

From feral hog to cape buffalo and everything in between, the 500 Auto Max out of Big Horn Armory’s AR500 is, in short, versatile enough to take down whatever you’re hoping to hang on the wall.