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510 Grizzly AR: Maximize Your AR-15


Photos by Kenda Lenseigne

There have been many attempts to maximize the potential of rounds that fit into the AR-15’s magazine. Want to shoot targets way the hell out there? 

6 ARC is the latest hot ticket solution, and if you want something with a longer track record and the ability to hammer medium game, the 6.5 Grendel should be on your radar. 450 Bushmaster and 500 Beowulf hit like the fist of an angry god, but if you want to hurl the biggest slugs, then the 510 Grizzly is worthy of your attention.

The most jaundiced of readers sitting at the back of the room are probably yawning and scratching their balls now, about to complain about yet another boutique cartridge no one asked for. Fair enough — no one’s forcing you to buy it, sprinkle tits. For the rest of us who are interested in cool ballistic innovation, here’s the skinny.

While she might tell you it doesn’t, size matters. The ADM lower came from one of our SBR’d work guns, so it’s been ridden hard and put away wet.

Originally presented as a case study at the National Defense Industrial Association conference in Austin, Texas, in 2022, the 510 Grizzly project was a collaboration between Tom Bowers (Bowers Suppressors), Mike Rintoul (Grizzly Cartridge Co.), and Howard Kent (nP Technology). 

Conceived as two cartridges using the same case, one had an overall length of 2.26 inches while the other maxed out at 2.80. Astute readers will no doubt note that these are the OALs of 5.56mm and 7.62 NATO, enabling the AR-15 to launch mighty 0.510-inch diameter projectiles, rather than the feeble 0.499-inch bullets kicked out by the Beowulf, and the AR-10 to propel the heaviest 50 BMG bullets, albeit at reduced velocities. 

All joking aside, while too long to be fed from a magazine, 647-grain M2 ball can be stuffed inside a Griz case and single-loaded into your favorite M4gery, while anything that can be run through a 505 Gibbs or 500 Jeffery can be crammed into a regular mag. 

Other bullet options range from 350-grain copper hollow points from Cutting Edge, 420-grain solid brass plinkers, and 425-grain nP Technology frangible slugs. Our test ammo loaded to AR-15 mag length consisted of a 570-grain Hornady soft point, designed to leave the muzzle of an 8-inch upper at a just-subsonic 1,080 feet per second.

The Woodleigh FMJ version of this round managed to penetrate in a straight line through 108 inches of ballistic gel, which is ideal should you wish to kill an enemy who is hiding behind the ass-end of a Cape buffalo.


The genius behind the 510 Grizzly is Mike Rintoul of Grizzly Cartridge Co, based in Rainier, Oregon. A mechanical engineer by training, he spent time in the oil fields of Alaska where humans rank a distant second on the food chain to large bears, so he quickly gained an appreciation for handguns that sling big bullets at moderate velocities. It’s no surprise then that the 510 looks like a giant handgun round. 

The monstrous size of the 510 Grizzly really becomes apparent when you look down the muzzle.

The 510 Grizzly uses a cut-down 300 Winchester Short Magnum case, itself based off the legendary 404 Jeffery. Its 0.532-inch rim diameter makes it the same as most magnum cartridges, but the 0.555-inch base diameter makes it a rebated rim design with a minimal amount of body taper, allowing it to feed from unmodified AR-15 magazines. 

A 570-grain bullet at 1,080 fps might seem like it would be adequate for just about any problem you might encounter, but according to Rintoul, it’s actually just loafing along like your underperforming teenage son. “We’ve loaded it up to 1,500 feet per second, and it still stays under 62,000 psi.” At that velocity, your lil’ AR-15 SBR kicks out more muzzle energy than a full-sized 308 Winchester. 

Of course, due to the fat case, magazine capacity is considerably reduced as the cartridge column shrinks from a double stack to a single row. Standard 30-round magazines have enough room to fit 10 50-cal cartridges (but nine rounds to feed reliably), while 20 rounders accept seven. For grins, we tried to see how many we could stuff in a Magpul 40 rounder, but the ridge molded into the front of the mag body prevented even a single shell from entering.  

One problem that all large-diameter cartridge designs must overcome is increased bolt thrust, due to the greater area of their case head that’s in contact with the bolt face. 

The AR-15 bolt already has several known weak areas, and this combined with the comparatively enormous case head diameter meant that simply opening up a regular 5.56 bolt was a non-starter. Instead, a 243 WSSM bolt and carrier from KAK Industries was used, proving to be a drop-in solution. This bolt has several design features that allow it to hold up. 

For a start, the bolt body diameter is increased from 0.530 inch in the case of a 5.56, to 0.613 inch, while the cam pin remains the same size. This goes a long way to addressing one of the key failure points in AR-15 bolt design, namely breaking at the cam pin hole. Another mode of failure is sheared locking lugs, and to mitigate this problem, the bolt is enlarged to 0.850 inch (versus 0.750), and the lugs themselves have a wave form to eliminate stress risers. 

As you might expect on such a stubby barrel, a pistol-length gas system is used, and bullets spin at a comparatively rapid pace of one turn in 8 inches. To accept the single column magazine and fat bullets, a lone feed ramp is cut into the barrel extension, rather than the usual pair. The muzzle had an adapter for a Bowers Group Vers 50 suppressor, and because we’re civilized human beings, we naturally took up the implicit offer.

There’s something a little comical about a 12-inch-long can installed on an 8-inch barrel, but the Vers 50 is one of the few suppressors that’ll pass a half-inch projectile without feeling like an anvil hanging on the end of the gun.

Unlike most 50-cal cans, it’s not rated for 50 BMG, but rather occupies the same rarefied space as monster big game rifles and tiger stripe Desert Eagles, so if you need a silencer for prey species up to and including velociraptors, then this might just be it.


Bolting up a Leupold Mk8 to a short-range brawler would be ludicrous, so in keeping with its role, we installed a DI Optics red dot and hit the range. Recoil is akin to shooting a 20-gauge shotgun with field loads — not uncomfortable, but you definitely know you’ve dropped the hammer on something significant. 

Standard AR-15 BCG right, Grizzly left. Note the comparative sizes of the bolt body and face.

No doubt, the suppressor has a little to do with this, but after shooting the Griz with both a bare muzzle and the can installed, we can say that recoil reduction isn’t as pronounced as we’re used to. The balance of high-velocity gas opposing a high-velocity projectile is skewed due to the size of the monster bullet. 

Another thing we noticed — despite the length and volume of the Vers 50, things are still pretty loud. No matter which way you slice it, that’s a big hole down the center of the can, and as a result, gas manages to find a way out while still retaining some speed. 

Couple this with port pop, and you don’t get the kind of results you’d see in, say, a 300BLK. Effect on target was — how shall we put it — quite noticeable. We actually wound up breaking one of our targets at its mounting bracket during a mag dump, which was surprising, as it had held up to many hundreds of 12-gauge rounds, as well as 223 and 308.

So what role does the 510 Grizzly fill? The obvious one would be as a short-range hunting tool with bullets in the 450-grain range loaded to 1,700 fps — it would hammer any species in North America, including the big bears. We had planned to take this one to put the hurt on a bunch of hogs, but alas, the stars didn’t align; hopefully, we’ll still make this happen, and you’ll see the results on RecoilTV.

DI Optics’ TRF-1 red dot has all the features you want in a tube sight — 30K hours of battery life from a AA, seven NVG settings, and an integrated mount.

There’s also one very specialized job in which we think it would excel. With 7+1 rounds of 425-grain frangible ammo at 1,300 fps, an 8-inch barreled upper would be superb for breaching doors, displacing a shotgun in the stack and almost doubling its capacity. 

In addition, whereas most departments’ and units’ SOP is that the breacher doesn’t make entry, due to having to stow their gear and having a suboptimal weapon in their hands, with an SBR they’re still in the fight — same manual of arms, same optic, and anyone hit with a 425-grain bullet is going to feel it. 

So far, we’ve only touched on the small frame version of this project. The next steps are to see how the AR-10 version performs, starting with 647-grain ball bullets and seeing how far we can go. Watch this space … 

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