Review: The Big Horn Armory AR500 : 3700ft/lb of Autoloading Power!

Rusty S.
by Rusty S.

Big Power from Big Horn

The AR platform keeps attracting new cartridges like moths to a flame. While it may be tiresome to see the same basic profile over and over again with new cartridges and configurations, there is sound rationale behind this phenomenon. The platform is extremely modular, accurate, and adaptable to many different disciplines within the shooting world.

That base being covered, I had a chance to try out a pre-production model of Big Horn Armory’s latest project, the AR500, chambered in the .500 Auto Max. (For previous TFB coverage on this platform, check out here and here). Big Horn has teamed up with Buffalo Bore to create a rimless, autoloading version of the .500 S&W Magnum in order to bring dangerous game capability to the AR platform. Being so new, does the new rifle and cartridge work well together? Can the AR platform handle shooting 440 grain bullets as fast as one can pull the trigger? Read on…

AR500 Specifications:

  • 1:24 twist
  • 18″ Barrel
  • adjustable gas block
  • full picatinny rail
  • M-Lok forend
  • adjustable buttstock
  • flash suppressor
  • no sights
  • 10 round magazine
  • hard plastic case
  • Price: $1999.00

My additional findings:

  • Weight: 9lbs, 1.7oz unloaded, 10lb, 5.3oz with a 10 round loaded magazine
  • Carbine Length Gas Tube
  • Thread Pitch: 3/4×28
  • Magazine Capacity: Production models will have 5 and 10 round magazines

Inititial Impressions:

The BHA AR500 came in a very sturdy plastic hard case with custom cut foam insert, the same that they ship their lever action rifles in. Inside the box was a preproduction model of their AR500. Immediately apparent was the lack of flash hider or muzzle brake, as well as a magazine well block held in place by roll pins, akin to the 9mm Colt SMG but on a much larger scale. This modification was due to Big Horn settling on using modified .50 Beowulf magazines which fit in AR15-sized lower receivers. There have been 3 changes to the magazine of the AR500 that I know of. Initially, the AR500 fed from modified SR-25 pattern magazines. Then there was a (rather ungainly looking) prototype hybrid system seen on the model that American Rifleman tried out. The model I received reflected the move to AR15 platform-sized magazines. Production lower receivers will be AR10 sized, but with the magazine well machined to accommodate the smaller magazines. No magazine well block will be needed. The use of small frame magazines in large frame lowers has been used recently for larger cartridges in the AR platform. Wilson Combat has come to a similar conclusion with the .458 Ham’r, and CMMG has utilized a similar system in their “Mutant” mid-size platform.

The Magazine block
Pins to hold mag block in place

This being a preproduction gun, the castle nut was not staked, and the furniture configuration was not final. I have been told that the buttstock will change to one with a robust recoil pad. Sharp-eyed TFB readers will recognize the lines of the upper and lower receiver as being from Ascend Armory, though Big Horn Armory does all the final assembly and machining. Ascend’s machined billet receivers are matched for superior fit and accuracy, and the ones selected by BHA are made from 7075. Moving on to the handguard, it had a full length rail at the 12 o’clock position. The shorter rail sections fore and aft with QD sling attachment holes on the 3, 6, and 9 o’clock position made for a very comfortable, mostly slick handguard.

Note the hybrid configuration of AR15 magazine and AR10 lower

The lower featured Ascend’s ambi selector switch and bolt latch and very nice anti-rotating trigger pins. The magazine release was RH only. The single-stage trigger broke cleanly at an average of 4lb, 5oz and engagement surfaces were polished. The upper receiver had a regular AR-10 charging handle and latch, and an ejection port appropriately enlarged to handle ejecting the portly .500 Auto Max cases. The direct impingement bolt carrier held an appropriate sized bolt head for the new cartridge. The 18″ barrel was attached via a barrel nut specific to the handguard. The handguard was in turn secured to the barrel nut via dual hex screws at the 3, 6 and 9 o’clock positions. There was no doubt this was a “Big Bore” gun, the muzzle was a gaping maw and I half expected to see Sean Connery or Roger Moore stroll into frame when looking down the bore!

Range Results:

I was determined to hit the range with the AR500, despite the nasty and unpredictable springtime weather one can encounter in the mountains. Thankfully, I did not have to hike the gun and all my equipment in, and was able to spend significant time having fun with (and exhausting the ammunition supply of) this rifle.

Ammo and accuracy:

The .500 Auto Max was developed from the .500 S&W Magnum specifically for this rifle. Per Tim Sundles of Buffalo Bore: Though .500 Auto Max cartridges will chamber and fire in .500 S&W revolvers and single-shot guns (the reverse is not true) there are a few key differences. The .500 Auto max has a slightly smaller rim and has no roll crimp, as it headspaces on the case mouth. I should note that Big Horn Armory and Buffalo Bore were kind enough to provide me with 4 different loads for this rifle:

  • Handloaded Sierra 400gr JSP
  • 440gr Hard-Cast Lead Flat Nose Gas-Check
  • 350gr Sierra JHP
  • Barnes 275gr XPB
L to R: 275gr Barnes, 440gr Buff. Bore, 400gr Sierra JSP, 350Gr Sierra JHP

Equipped with my Magnetospeed V3 chronograph, I was very interested to see what sort of velocities I could get out of the AR500’s heavy loads. Keep in mind that the ammunition I had on hand was from preproduction lots. Apparently Buffalo Bore has fine-tuned the loads and as of writing this piece they have achieved slightly higher velocities.

Here are my velocity and energy results by cartridge, all velocities in ft/s, all muzzle energy in ft/lbs:

  • Barnes 275gr XPB-Max: 2414, Min: 2334, Avg: 2383 ME:  3467
  • Sierra 350gr JHP-Max: 2214, Min: 2141, Avg: 2186, ME: 3713
  • Sierra 400gr JSP-Max: 1987, Min: 1920, Avg: 1953, ME: 3387
  • Buffalo Bore 440gr HCFNGC-Max: 1866, Min: 1828, Avg: 1844, ME: 3322

Certainly the .500 Auto Max is capable of being a devastating dangerous game cartridge out of an 18″ barrel. As of the time of writing this article, Buffalo Bore has 3 loads available with very detailed data (click on a specific load for very detailed tables) and more loads are in the works. Current price per box of 20 rounds is $67.

Ready for accuracy testing

For accuracy testing, I chose to equip the AR500 with an older generation Leupold 1.5-5x20mm MR/T. This choice of scope was due to that 500 Auto Max is certainly powerful out to 300 yards, but rapidly decreases in utility further out. I shot the rifle off a bench, bagged for support front and rear. Accuracy results are for 5-shot groups at 100 yards, measured center-center:

  • Barnes 275gr XPB: 2.13″
  • Sierra 350gr JHP: 2.3″
  • Sierra 400gr JSP: 1.719″
  • Buffalo Bore 440gr HCFNGC: 1.78″

The very best groups I achieved were from the 400gr and 440gr loads, at .9″ and 1.19″ respectively. This is very good accuracy for a dangerous game rifle. I have been told by Big Horn Armory that 1.5 MOA seems to be the accuracy norm for this rifle and cartridge, and that the 1:24 twist rate does indeed favor the 400gr and 440gr bullets.

A typical group. Impact to the right, outside of the box, was for initial sight-in of this grain weight.
Cold-bore accuracy was excellent
Dynamic shooting, reliability, and recoil:

Shooting this rifle off the bench without a brake resulted in stout recoil, but nothing like shooting a .416 Rigby or .450/400 NE. I would liken it to that of a lever action .45-70. The report of the rifle was not particularly jarring, but the bolt and recoil spring sounded like a old squeaky screen door slamming shut. If such a thing bothers you, I would definitely suggest JP Enterprises’ Silent Capture Spring for this rifle. Moving out to steel targets at 500 yards, the AR500 was able to find its mark and authoritatively clang the target repeatedly with an appropriate hold. This was during a windy flurry of snow and sleet moving in. The heavy rounds didn’t seem to care much about the swirling winds at that distance.

The 500 at 500? No Problem!

Moving over to a pit with moving targets, I tried out the AR500 in a few different scenarios while kneeling and offhand to better simulate dangerous game hunting or bear defense at 50 yards. Though the rifle’s muzzle tracks well when swung to follow a moving target, I felt that the overall balance is a but front-heavy due to the large diameter barrel. This can be easily remedied by the end user with a different buttstock however. If this was my personal rifle, I would definitely augment it with a FAB Defense GL-Shock stock for additional weight as well as recoil reduction. With the scope on 1.5x magnification, it was easy to keep all 7 rounds within 5″ at 50 yards when shot offhand as fast as I could pull the trigger. Definitely a plus for bear defense or on charging game, and much quicker (and faster to reload) than using a manual repeater or double rifle in a similar scenario. An interesting note: With outside temps in the mid-20’s, during rapid-fire strings the barrel never got too hot to touch.

This car roof shot offhand at 150 shows the good accuracy capability of the .500 Auto Max
The aftermath of slinging a LOT of lead downrange 400 grains at a time.
Old meets new

Being that the rifle did not come with a magazine, I tried it out with standard AR15 magazines. I found the best reliability with straight profile magazines such as Magpul’s 20 rounder gen2 Pmags and 10-round DH Tactical magazines. I could fit 7 rounds in the 20-rounders and 4 rounds in the 10-rounders. The 30-rounders did not function. During rapid fire drills, I could reliably fire 7 rounds as fast as I could pull the trigger. It’s good to have this ability to function with stock AR15 mags in case one loses the optimized magazines or forgets to bring them on a hunt. Ejection was regularly 3 feet to my 4 o’clock. The rifle was mostly reliable, though I did encounter one light primer strike on round #30, which was a Barnes 275 XPB. Around round #130, the bolt stopped going fully into battery. I stripped the rifle, and relubricated the bolt and bolt head. The rifle then resumed functioning until I exhausted the 160 round ammunition supply. Keep in mind one is putting a LOT of lead downrange with each shot, and 130 rounds in a single range session is almost unheard of with a dangerous game rifle.

That's one big round...
Ready to chamber
Bad things happen to lead when this gun starts to run dry...
The case got a little messed up when the bolt locked up
The cases don't get too badly beat up on ejection during normal function.

Overall Impressions:

With the AR500, Big Horn Armory has created a viable autoloading and mostly ambidextrous dangerous game rifle that can handle anything from grizzly bears to Cape buffalo, or the occasional charging semi truck. Reasonably priced as far as such rifles and ammunition go and offered on the highly customizable AR-10 platform, this rifle worth consideration if such a capability is needed. As far as ARs go, this certainly ranks as one of, if not the most, powerful configurations in an AR10 platform. Reloaders will enjoy the capability of using most .500 S&W components. For those who don’t reload, 3 factory loads are now available and more are in development. If one is considering this rifle, I would recommend reloading after building up a good supply of brass to achieve the best value and versatility. As far as the .500 Auto Max cartridge, this author hopes that the AR500 with an 18″ is not the only platform it will see. A “pistol” version with a 10-14″ barrel could enhance its portability as a purely defensive arm or open up the legal ability to use it in more “short range” hunts. I would also be curious to see if the cartridge could be adapted to smaller suitable pistol platforms besides the AR.

The AR500 offers devastating and accurate mid-range power from the AR-10 platform in a portable package, and from my experience it delivers on that promise quite well.

For more information or to order, please visit:

Big Horn Armory

Buffalo Bore

Thanks to Big Horn Armory and Buffalo Bore for the opportunity and the ammunition!

Thanks to Hughston Shooting School for range time and logistic support!

Rusty S.
Rusty S.

Having always had a passion for firearms, Rusty S. has had experience in gunsmithing, firearms retail, hunting, competitive shooting, range construction, as an IDPA certified range safety officer and a certified instructor. He has received military, law enforcement, and private training in the use of firearms. Editor at

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2 of 19 comments
  • GaryOlson GaryOlson on Apr 13, 2018

    "Adjustable gas block". What type of adjustment device? With that amount of oomph, I'm curious about long term buildup and maintenance.

  • Ozzallos . Ozzallos . on Apr 14, 2018

    Would rather have that lever, thnx :)