Think the AR platform is only good for target shooting, self-defense, predator and varmint hunting? Think again with the San Tan Tactical STT-15 Big Bore PILLAR Billet Upper Receiver. For starters, the STT-15 Big Bore PILLAR upper receiver is incredibly light coming in at 8.2oz! It features an enlarged ejection port to freely pass .458 SOCOM all the way up to .50 BEOWULF. We designed the STT-15 Big Bore PILLAR upper receiver to overcome the shortcomings we saw in the current offerings. It needed to be lighter, stronger, and different, and all while looking good doing it. We weren’t going to settle for just hacking another receiver out of a block and calling it good!

Why a big bore upper? Well, besides being a great gun for casual shooting, competitive shooting, and self-defensive applications, the AR makes a fantastic hunting rifle. More and more deer hunters are carrying AR rifles and the platform has become one of the top choices for hog hunters. But what about larger stuff? What about pursuing creatures that can turn the tables and in an instant, turn the hunter into the hunted? Is there a place in the world of dangerous game hunting for big-bore AR’s? In the world of self-defense AR rifles?

The AR-15 design limits the length of the cartridge, so the only way to increase the power level is by going bigger diameter. That’s good, as heavy bullets at moderate velocity are well proven in the hunting fields. Why not just use a powerful bolt-action hunting rifle? In a survival circumstance, we must assume that the world will be a dangerous place. A magazine-fed, semi-automatic rifle in your hands gives you the best hope for winning a fight, whether it be against dangerous animals or two-legged predators. Of course, bigger cartridges lower magazine capacity compared to 5.56 NATO, but the magazines will hold more ammo than a bolt-action and reloads will be much faster. For example, the .458 SOCOM uses unmodified, metal .223 Rem. magazines. The .223’s 30-round magazine holds 10 of the big boys. You also will have a lot of fight-stopping power with each of these bullets. Don’t let anybody tell you that size doesn’t matter in a fight or when hunting. It does. Big bullets make bigger holes, smash more structure and rip up a lot more vital stuff than little bullets. 

The most common AR chambering, the 5.56/.223, is somewhat anemic compared to popular hunting calibers like the .30-06 and .300 Winchester Magnum. The AR-10 platform chambered in .308/7.62×51 offers considerably more knockdown power, and other cartridges like the 6.8 SPC and .260 Remington are versatile hunting rounds that can be chambered in AR rifles. But again, what about an even larger game such as bears, bison and other large, heavy, potentially dangerous animals? Is there an AR suitable for that class of game? In truth, there are many. There are several options for hunters looking to pursue the largest game with their AR and plenty of effective cartridges. The good news is that many of these cartridges are sufficient on a wide variety of game, so whether you’re tackling big hogs or brown bears, one rifle will do it all. Versatility is, after all, one of the hallmarks of the AR platform.


The STT-15 Big Bore PILLAR Billet Upper Receiver will accommodate a variety of big bore calibers and attaches to an STT-15 lower or any AR15 lower you currently have. Below we outline a few of the more popular calibers and their advantages over the traditional 5.56x45 commonly used in AR15’s.

.450 Bushmaster
The concept for the .450 Bushmaster was developed by Col. Jeff Cooper. Cooper was dissatisfied with the small-diameter 5.56x45 NATO round of the AR15 and recognized that a big bullet is a good thing when shooting big game. He thought that the perfect rifle for most “general” big-game hunting would be a semi-automatic variant, larger than .44-caliber and capable of taking big game out to 250 yards. He called this the “Thumper” concept.

Tim LeGendre, of LeMag Firearms, developed the cartridge and named it the .45 Professional. He licensed the rights to Bushmaster Firearms International with Hornady working in conjunction with Bushmaster to develop the ammo. Hornady modified the case a little so it would work better with Hornady’s proven 250gr SST Flextip bullet. With LeGendre’s permission, the name was then changed to .450 Bushmaster, and the cartridge was introduced in 2007. The current load from Hornady for .450 Bushmaster uses a 250gr FTX bullet with a factory advertised muzzle velocity of 2,200 fps from a 20-inch barrel for 2,680 ft.-lbs. of energy. Remington loads it with a 260gr Premier AccuTip projectile at 2,180 fps. Muzzle energy is 2,744 ft.-lbs.

.458 SOCOM
After the fighting in Mogadishu in 1993, some of the participants were disappointed in the performance of the 5.56 NATO cartridge and wanted something with a bit more knock-down power for the M16/M4-style rifles. The result was this cartridge. The .458 SOCOM was introduced in 2002, and while it didn’t gain widespread acceptance as a military round, it has found a home with civilians. The cartridge was designed to be 100% compatible with the M4 platform. This included the buffer, buffer spring, NATO magazines, and many aftermarket .223/5.56 magazines. A standard 20-round 5.56mm magazine can hold seven .458 SOCOM rounds and a standard 30-round 5.56mm magazine can hold ten .458 SOCOM rounds. That’s a lot of firepower available in a hurry.

Most hunting loads generate energy levels north of 2,200 foot-pounds, making this an excellent choice for bear defense. One big advantage of .458 diameter is there is a wide selection of bullets on the market with bullet weights ranging from 140 to 500 grains. This cartridge probably is best suited to bullets in the 300- to 400-grain range. The .458 Socom is a powerful cartridge capable of taking any animal in North America and designed by guys who have been to battle and wanted something better. This cartridge deserves a look from anybody serious about survival, hunting or defense.

.50 Beowulf
The .50 Beowulf was developed by Alexander Arms and introduced in 2001. It was the first of the big bore cartridges designed for the AR-15. The cartridge is based on a lengthened .50 Action Express and modified to function in AR rifles with the bullets being .500-inch, the same as the .500 S&W Mag. The Beowulf ups the AR ante when it comes to power. Like the .458 SOCOM, it began life as a cartridge designed to offer a horsepower upgrade over the standard 5.56×45 cartridge, and it does so stunningly. The cartridge found favor with military and law enforcement agencies because it wouldn’t bounce off window glass in vehicles, but it certainly has applications for hunting large, dangerous game as well. 


For starters, the cartridge generates between 2,200 and 2,800 foot-pounds of energy and places a half-inch hole in whatever it strikes. The cartridge has plenty of power for short to moderate range hunting of species like bear, elk, moose, large feral hogs and even bison. The 400-grain Beowulf has 627 percent more bullet weight and an unexpanded diameter that is larger than a .223’s fully expanded bullet. Expanded, the Beowulf bullet has a 123-percent larger frontal diameter than the .223’s expanded bullet. The Beowulf has almost three times as much muzzle energy as the .223 Rem. and more than four and a half times the momentum. This is a serious step up in power for the AR-15 platform.

These cartridges are very specialized, but they do have a strong place in prepping, hunting or for defense. I would consider buying an STT-15 Big Bore PILLAR Billet Upper Receiver that you can build to use with your current AR-15 lower. Any of these three cartridges will make foraging much more successful and safer. They also can be handy in some defensive situations. If you have a problem who’s hiding behind a barrier or something that needs breaking, like a bad guy’s car engine, these brutes can get it done better than any other AR-15 cartridge. That’s gotta count for something, right?