Big Bore Cartridges for AR-10s & AR-15s

By Dean Roxby

Do you love big bore rifles? Somebody must like them, based on the variety of large caliber cartridges introduced in the last decade or so.

Years ago, the AR-15/M16 rifle was spoken of rather poorly by some, due to the small cartridge and “tiny” bullet. Now with caliber choices up to .50 inch, those complaints are rarely heard.

The real beauty of the AR family of rifles is their modularity. It is extremely easy to change virtually every piece of your rifle, if you so desire. This allows for all manner of customization. In particular, a change of cartridge is accomplished by a simple swap of the upper receiver/barrel assembly and perhaps a bolt and magazine swap. This applies to both the massively popular AR-15 platform and the AR-10 as well.

The trend of big bore AR cartridges is something that has intrigued me for some time now. Let’s take a look at what is out there. Please keep in mind that this article is merely an introduction and not meant to be a contest to determine “the best.” Attempting to do so would be an instant failure, as various rounds have different intentions. For example, the Am-Tac 416 Hushpuppy was meant for suppressed use, while the mighty .500 Auto Max is for hunting large, tough animals. It would not be reasonable to expect the Hushpuppy to match the Auto Max in power. To answer which is “Best,” we would need to clarify “Best at What?”

If you buy a complete rifle, it should be set up and ready to run—Well, hopefully. Buying an upper for your existing lower is certainly doable. After all, that is the genius behind the AR system. You may want to change the buttstock to one beefy enough to handle the recoil. Collapsible stocks are best avoided.

If you want to do your own start-to-finish build, you will HAVE to open up the ejection port for most if not all of these rounds to allow the fired case to be ejected cleanly.

The good news about magazines is that standard AR-15 or milsurp M16 mags work fine in most cases. But of course the capacity is greatly reduced, due to the larger case size. A 30-round mag will hold about 10 of these big bore cartridges, and they will be single-stacked vertically, rather than staggered as with the 5.56 round. This refers to the AR-15 group. This is not an issue with the AR-10 group of cartridges, as the case body diameters are not so dramatically different.

Beck Ammunition, 510 Beck. From the SHOT 2016 Media Day.

These are grouped into rounds for the AR-15 platform, then for the AR-10 and in order of increasing caliber. (The Wilson Combat .458 HAM’R is a hybrid, so is dealt with separately.)

Chamber pressures are deliberately kept low with the AR-15 group, due to the issue of bolt thrust. The total force pushing rearward on the bolt face is a combination of pressure per a given area (pounds per square inch), multiplied by the area of the base of the round. If the cartridge base is larger, then the chamber pressure has to be lowered in order to keep the total bolt thrust within safe limits.

The Wilson Combat .458 HAM’R is unique in that it uses a non-standard hybrid receiver. The BCG is also ¾-inch shorter than a standard AR-10 one. The receiver uses the smaller AR-15 mag well, yet the bolt and barrel extensions are larger AR-10 dimensions. Because of that, this carbine-size gun can handle chamber pressures of 46,000 PSI and generate over 3000 FPE. The downside to this is that the ability to swap uppers is lost. If you want a HAM’R, you buy the complete rifle.

Some of these loads use short-for-caliber pistol bullets, while others use long-for-caliber rifle bullets. Generally, a short, light, pistol bullet starts out faster but sheds velocity sooner. Conversely, a longer, heavier rifle projectile will move slower but will not loose speed as rapidly, due to the better sectional density. Keep this in mind when comparing data from different sources.

To keep this article manageable, the author has limited the list to rounds with correct headstamps, keeping it to a handful of proprietary or “boutique” rounds. It simply would not be possible to keep track of every wildcat cartridge. Even still, it is likely that other cartridges have been overlooked. The author learned about one of them as the article was being prepared. If your favourite big bore AR round is not here, please forgive his error.

Most of these rounds originate from small companies. A few are family businesses operating out of a garage or barn. I view these Mom-and-Pop shops as true entrepreneurs, and I wish them much success.

The majority of cases are formed by Starline Brass. Starline really must be commended for helping the shooting sports community as they do. Three ammunition loading companies stand out as well. Buffalo Bore Ammo, SBR Ammunition and Underwood Ammo load high-quality ammunition, starting with Starline Brass and various premium bullets.

AR-15

.375 SOCOM

Created by Tony Rumore of Tromix Lead Delivery Systems in 2013, this is simply the .458 SOCOM case necked down. (The .458 SOCOM is listed below.) Correctly headstamped brass, as well as factory ammunition, is available from SBR (Southern Ballistic Research) Ammunition.

.416 Hushpuppy

Appearance-wise, this may be the most intriguing looking round in this article. Most of the other cartridges covered are going for maximum power, so have the largest case that can be held in an AR-size mag. This round is meant to stay subsonic to avoid making a sonic crack when fired. This is accomplished by using a small capacity case and a long spire point 450gr bullet. The case is a shortened and reformed 50AE case. The result is a round with more copper bullet than brass case. The look has been compared to a lawn gnome! This was first debuted at the 2014 SHOT Show. Unfortunately, it seems to be dormant now as any current contact info for its creator, Am-Tac, could not be found.

.450 Bushmaster

Based upon the .284 Winchester case, this straight-walled case uses a rebated rim with a .473-inch diameter, the same as the .308 Winchester family. Designed by Tim LeGendre of LeMag Firearms, it was originally known as the .45 Professional. Bushmaster and Hornady further developed it, shortening the case slightly to 1.700 inches long. Note that it uses .452-inch pistol bullets and not .458-inch rifle bullets. As it is a rimless, straight-walled case, it headspaces on the case mouth; the same as most semi-auto pistols. As such, reloaders must use a taper crimp, not a roll crimp.

.458 SOCOM

Created by Marty ter Weeme of Teppo Jutsu in late 2000 as a result of poor terminal ballistic performance of the 5.56 NATO round during a battle several years earlier in Mogadishu, Somalia (the “Black Hawk Down” battle). Based on a lengthened 50AE case with a rebated rim (.473-inch, same as the .308 Winchester family) and necked down to .458 at the mouth. Teppo Jutsu contacted SBR to supply factory ammunition early on. SBR has a wide selection of loaded ammo, as well as correctly headstamped brass. Starline also makes brass. The name SOCOM refers to Special Operations Command. This round has become very popular recently.

.499 Leitner-Wise

An early entry into the big bore AR game, the .499 LW is now effectively dead, as the restructured LWRCI company no longer supports it. Named for Paul Leitner-Wise. Based on a lengthened .50 Action Express case with a rebated rim, it is similar to the far more successful .50 Beowulf. For a short time, the US Coast Guard tested this round for intercepting drug smugglers.

.50 Action Express

In the mid- to late 1990s a few experimenters built guns on this magnum pistol cartridge. In this case, the rifle was designed around an existing cartridge, rather than a cartridge being created for a rifle. The cartridge was first introduced with the mighty Desert Eagle semi-auto pistol from Israel. The original idea behind the 50AE was to allow for a simple barrel swap between it and a 44 Smith & Wesson Magnum barrel in the DE handgun. The rim is the same diameter in both rounds, making caliber changes a simple matter. (The case bodies are noticeably different sizes, resulting in the 44 being a rimmed case and the 50AE being a rebated design.) Unfortunately, the standard unmodified case has a large rim (.514-foot diameter), which weakened the bolt lugs and extractors. Parts breakage was common. It is seldom encountered now in ARs.

.50 Beowulf

Designed by Bill Alexander of Alexander Arms, the .50 Beowulf is based on a lengthened 50AE case, with a reduced diameter rim similar, although not identical, to the 7.62×39 rim diameter. Aside from a few one-off experimentals, this was the first of the big bores specifically created for the AR-15 platform. Dies and brass are available from Alexander Arms.

.502 Thunder Sabre (TS)

Designed in 2004 by Robyn Church of Big Bore Canyon/Cloud Mountain Armory/Force Ten Firearms in Oregon, the rights were later sold to R&J Firearms of McMinnville, OR. The .502 TS is a 50AE case with the rim diameter reduced to .445 inch (7.62×39 rim diameter) to avoid the parts breakage issue mentioned above. As a .502 TS round is quite a bit shorter than a typical .223 Remington or 5.56×45 round, the mag has a filler block inside it in order to hold the rounds to the rear of the mag.

AR-10

.375 Raptor

Created by Arne Brennan of North American Sportsman, LLC, this round follows the .45 Raptor. The .375 Raptor is simply a .308 case opened up to .375 diameter, shortened slightly and with the shoulder angle changed to 35 degrees. This allows for more positive headspacing. Correctly stamped brass is available from Red Stag Ammunition.

.45 Raptor

This is an interesting design, introduced in 2014. The .45 Raptor is essentially a rimless .460 Smith & Wesson magnum cartridge. It uses .460 Smith & Wesson load data and .460 Smith & Wesson dies, along with a .308 Winchester shell holder. It can be formed from .308 Winchester or 7.62 NATO brass that has been shortened to 1.800 inches and opened up to accept .452 bullets.

Brass is made by Starline Brass and headstamped RSS (Raptor Shooting Systems). Loaded ammo from Underwood Ammo.

.45-70 Auto

Phoenix Weaponry of Longmont, CO, introduced this one at the 2018 SHOT show. They created a rimless version of the classic .45-70 Government cartridge of 1873. It uses standard .45-70 dies and data, along with a .308 Winchester shell holder. The sample cartridge the author saw at SHOT was made from a lathe turned .45-70 Government case. Properly headstamped cases are in the works now.

.50 Krater

The .50 Krater round is based on a 300 RUM case. It uses .502-inch-diameter pistol bullets, the same as .500 Smith & Wesson or 50AE. Brass and ammo are available from Outdoor Shooters Supply/Red Stag Ammunition.

.500 Auto Max (AM)

Created by Big Horn Armory of Cody, WY, this is a rimless version of the thunderous .500 Smith & Wesson Magnum revolver round. Although the rim has been removed, the extractor groove dimensions remain the same, so reloaders will use the .500 Smith & Wesson shellholder. As BHA president Greg Buchel explains: “The extractor groove is very deep and allows using the same shellholder, even with the rim removed. The shellplate is cut for the extractor groove diameter.”

Originally named the Auto Mag, the name was soon changed to Auto Max. They may not be exaggerating. The BHA website lists a number of loads, most of which exceed 4000 FPE. One is almost 4600 FPE! That is suitable for African Dangerous Game. Currently, this is the most powerful round for the AR series. The .500 AM can be reloaded with standard .500 Smith & Wesson dies and reloading data. As with other rimless straight-walled cases, it must be taper-crimped, not roll-crimped. It is worth noting that while the .50 Beowulf is limited to about 33,000 PSI chamber pressure, the .500 AM can run up to 60,000 PSI. Underwood Ammo and Buffalo Bore load this round.

.510 Beck

First shown at the 2016 SHOT Show, this round seems to have vanished. Created by Beck Defense of TX, it got a lot of interest at Media Day. It features a .473-inch-diameter rebated rim on a straight-walled case. The website still mentions it being available 1st quarter 2018, yet emails go unanswered. I hope this one returns, as it seemed to have a lot of potential.

Hybrid Receiver

.458 HAM’R

Designed by Bill Wilson of Wilson Combat, the .458 HAM’R is very similar in dimensions to the .458 SOCOM round. However, they are NOT interchangeable. The Wilson website states: THE .458 HAM’R AMMUNITION IS LOADED TO MUCH HIGHER PRESSURE THAN THE .458 SOCOM AND IS NOT SAFE TO FIRE IN A .458 SOCOM CHAMBERED RIFLE.

The HAM’R system uses a hybrid rifle, similar in size to an AR-15 but uses a modified AR-10 bolt and barrel extension. This allows higher chamber pressure and improved ballistics. It is claimed that it fires a 300gr bullet 200 fps faster than the SOCOM. Interestingly, it uses .458 SOCOM reloading dies but is set a .040 inch longer to prevent a HAM’R round from being chambered in a SOCOM rifle. The cases are correctly headstamped to avoid mix-ups.