Underwood 500 Auto Max 350gr XTP JHP – Proven 20 Round Box

Description

Underwood 500 Auto Max 350gr XTP is what you feed a big bore semi automatic when the goal is a hunting bullet that actually opens. Underwood catalogues it as SKU 926, loading a 350 grain Hornady XTP jacketed hollow point to 2300 fps and 4112 ft-lbs of muzzle energy. Twenty rounds to the box, loaded in the United States.

• What 500 Auto Max Actually Is
• Underwood 500 Auto Max 350gr XTP Specifications
• Honest Notes on Big Bore Semi Autos

What 500 Auto Max Actually Is

The 500 Auto Max is essentially a rimless 500 S&W Magnum, developed so that the enormous 500 diameter bullet could be run through a semi automatic action rather than a revolver cylinder. Big Horn Armory built the AR500 around it. Ballistically it lands in the same neighbourhood as its revolver parent, but out of a rifle length barrel it gains velocity that a handgun simply cannot reach.

A 350 grain bullet at 2300 fps generates 4112 ft-lbs at the muzzle, which is more energy than a 30-06 and delivered by a projectile roughly twice the frontal area. For hogs, black bear, big whitetail in heavy timber and anything else inside 150 yards, that combination is decisive. This is genuinely capable big bore hunting ammunition rather than a novelty.

The bullet choice matters as much as the numbers. The Hornady XTP jacketed hollow point is a controlled expansion design with a serrated jacket and a swaged core. It opens reliably across a wide velocity band and holds enough weight to keep penetrating, which is exactly what you want when the shot might be quartering through the shoulder of a two hundred pound boar.

Feeding reliability is worth a word too. Big bore semi automatics are sensitive to overall length and case taper, and handloads that work in a single shot will not always cycle a gas gun. Factory 500 Auto Max ammo from Underwood is dimensionally consistent, which is the practical reason most AR500 owners stay with commercial loads rather than rolling their own. If you do reload, keep the brass separated by firing and watch the case mouths.

Underwood 500 Auto Max 350gr XTP Specifications

Underwood loads this in the United States and publishes velocity from a rifle length barrel. Shorter barrels will read lower. The XTP is a lead core bullet, so check whether your hunting area requires non lead projectiles before you head out.

• Manufacturer SKU: 926
• Caliber: 500 Auto Max
• Bullet weight: 350 grain
• Bullet type: Hornady XTP jacketed hollow point
• Muzzle velocity: 2300 fps
• Muzzle energy: 4112 ft-lbs
• Case: rimless straight wall, boxer primed brass
• Quantity: 20 rounds per box
• UPC: 816874023140
• Primary use: hog, bear and deer hunting inside 150 yards

Honest Notes on Big Bore Semi Autos

Rifle compatibility is the first thing to confirm. 500 Auto Max ammo is not interchangeable with 500 S&W Magnum, and a revolver chambered for the rimmed cartridge will not headspace it. It is designed for the Big Horn Armory AR500 and rifles built to that specification. Check your chamber marking, not the box, before ordering.

Recoil is real. A 350 grain bullet leaving at 2300 fps out of a gas gun is manageable because the action soaks up part of the impulse, but it is still a heavy push and a very loud one. Muzzle blast from a short barrelled big bore is punishing for anyone standing beside you, and proper hearing protection is not optional at the range.

Cost per round is the third caveat. This is specialty ammunition made in small runs and it is priced accordingly. Nobody plinks with Underwood 500 Auto Max 350gr XTP ammunition. Buy a box to confirm zero and function, then keep the rest for hunting season.

Finally, know your range limits. The 500 diameter bullet has a low ballistic coefficient and sheds velocity quickly, so trajectory past 200 yards drops steeply and wind drift grows. Inside 150 yards it is devastating. Beyond that, a flatter cartridge is simply the better tool, and being honest about that is part of hunting responsibly.

Storage and handling deserve a note as well. These are large, heavy cartridges and a full box weighs more than most people expect. Keep them dry, keep them labelled, and do not mix them into a general ammunition can where a 500 S&W Magnum round could end up alongside them. Confusing the two at the range is the kind of mistake that damages a firearm.

Underwood also loads heavy hitters in other chamberings we stock, including 450 Bushmaster 360 grain and 50 Action Express 300 grain bonded JHP. Full published data is on the Underwood Ammo product page.

Underwood 500 Auto Max 350gr XTP suits AR500 owners, guided hog hunters and anyone who wants revolver stopping power from a rifle action. MAG5 Firearms is a family owned FFL in Collegeville, Pennsylvania. Boxes ship where state and local law allows.