Genesis Arms Gen-12 12 Gauge AR-10 Shotgun Conversion Upper
Genesis Arms Gen-12
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Genesis Arms Gen-12 12 Gauge AR-10 Shotgun Conversion UpperGenesis Arms created the 12 gauge conversion for the AR-10 platform. Swap the upper, beef up your magazine catch, bolt catch (reverse compatible with regular AR-10s) and grab a Gen-12 magazine and your long-range AR-10 just became a versatile short range blaster.

Genesis Arms Gen-12 12 Gauge AR-10 Shotgun Conversion Upper
Despite Pacific Northwest weather doing what it does, the crowd was excited to try a Gen-12

I’ve been following Genesis Arms for a few years now and done my best to keep up with their advances. They recently hosted a range day just outside of Coeur d’Alene, Idaho so I made the drive from Oregon to catch up with them.

Finding out IPSC Pan-American Shotgun individual gold medalist Patrick E. Kelley was going to be there put some icing on the cake.

What Is the Genesis Arms Gen-12?

The platform is a work of genius and adds serious versatility to any DPMS-pattern AR-10 lower. The Genesis Arms Gen-12 was conceived originally as a breaching option for military and law enforcement use as a quick conversion that retains the familiar ergonomics and manual of arms of the AR. As you’ll see in our coverage of the event, the Gen-12 has proven to excel at more than just breaching.

With its massive load capacity, the 12 gauge round is easily one of the most versatile cartridges commonly available.

Genesis Arms Gen-12 12 Gauge AR-10 Shotgun Conversion Upper
The barrel of the Genesis Arms Gen-12 reciprocates slightly while a counter-weight helps reduce recoil.

How It Works 

The Gen-12 upper operates somewhat like Eugene Stoner’s AR-17 shotgun in that it has locking lugs on a rotating bolt and barrel extension, and cycles using a short-recoil system.

Upon firing the barrel travels a short distance rearward under recoil with the bolt and carrier. At the end of the barrel’s travel momentum carries the carrier further rearward, a cam rotates the bolt which unlocks. A spring returns the barrel to its forward position before the bolt can return with the next round.

The Gen-12 system is very fast and also absorbs some of the recoil typically felt by the shooter. Genesis Arms then took things a step further by installing a spring-loaded counterweight which decelerates the barrel movement, further reducing felt recoil (see video). The best part is this system runs reliably all the way down to 1 1/8oz birdshot loads at around 1,140 fps.

What’s The Drawback?

As you can imagine, a reciprocating barrel means that the Gen-12 could show sensitivity to muzzle devices, something breachers, suppressor owners, and casual shooters increasingly want. Genesis Arms developed a solution to this they call the “SBM” (Suppressors Breacher Model).

The SBM is a special mount that attaches to the handguard. I’ve seen 1,000 rounds go through a full-auto suppressed 7″ version of the Gen-12 without issue.

Genesis Arms Gen-12 12 Gauge AR-10 Shotgun Conversion Upper
The Genesis Arms Gen-12 SBM models feature a unique method of attaching muzzle devices. (handguard pictured does not contain a barrel)

Another challenge is that there is no “mil-spec” standard for AR-10 lowers. Genesis Arms opted to build off the DPMS pattern (rounded cut receivers) as they are commercially the most popular. But without any true standard not every receiver’s magazine catch is in the exact same place relative to top of the lower receiver.

Unfortunately, that means not all lowers function perfectly with a Genesis Arms Gen-12 upper despite the pins lining up. To remedy this Genesis has been compiling a list of known “go” and “no-go” lower receivers, and also offers their own which, of course, easily works with your .308 upper.

Cost is another drawback, but it’s also the nature of innovative products. Creating unique bolts, carriers, and other parts doesn’t come cheap. Complete firearms range from $2,449.99 to $2,859.99. Uppers range from $1,699.99 to $2,089.99

Genesis Arms Gen-12 12 Gauge AR-10 Shotgun Conversion Upper
The Genesis Arms Gen-12 Bolt and carrier assembly is understandably beefy.


Equipping your AR-10 lower with a Gen-12 upper requires using a Genesis Arms bolt catch, magazine catch, and magazine. The magazine is obvious. Genesis makes the only 12 gauge magazine that fits inside an AR-10’s magazine well. The magazine and bolt catches are required to ensure reliable operation because again, sadly, there is no “norm” for those parts on the AR-10 platform.

The best way to ensure that what you have will work (and isn’t an AR-15 part) is to use Genesis Arms’ parts. The good news is the magazine catch and bolt catch retain full functionality with your .308 setup. The magazines are unique, but fairly priced (as of June, 2022) at $22.99 for a five-rounder and $39.99 for a ten-rounder.

Real Owner Experience

In the four or so years I’ve owned my earlier-version (prior to the counter weight design) Gen-12 uppers I’ve enjoyed them thoroughly. The gun is chambered for 3″ shells and runs any reasonable 2 3/4″ shell without a problem. Due to inconsistency with the actual overall length of shotgun shells, not all 3″ shells will fit in the magazine. As a casual shooter though I rarely find a need for 3″ rounds.

Unlike imports that look like an AR, the Gen-12 truly is a conversion. That means you keep the same manual of arms, same trigger, grip, stock/brace that you love about your AR-10. It also means you can easily run 12 gauge just like an AR.

The recoil mitigation is incredibly effective which makes the shooter more effective and accurate. Follow-up shots are just as they would be on an AR. It’s also great for those who don’t care for extra recoil.

At the recent range day, 10-year old, 82-pound TJ was comfortable enough to put at least 50 rounds of 12 gauge downrange (see the video above). Yes he needed a little help supporting the weight of the gun, but the point is that the recoil was soft enough, and action forgiving enough that he had no qualms about emptying another magazine.

Who knows? Maybe if New York Daily News reporter Gersh Kuntzman had tried a Gen-12 first instead of an AR-15 he wouldn’t have been quite so bruised, horrified, and traumatized.


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  1. An interesting article, but it doesn’t fit my pistol. A rifle is the real answer to a gunfight. If I don’t have a rifle, I have a handgun, because I can’t have a long gun in my hands at that particular place in time. Just me. You’re mileage may vary.

  2. I’ve tried those tacticool shotgunms, I just ain’t as fast with them as a conventional stocked one.
    To each their own.

  3. I checked the no/no go list. Not very many “go” for that price point though that’s not at all their fault, that’s what we get for not having real AR-10 standards.

  4. Something I’ll say/ask is I thought that the max COAL for a pmag equipped LR308 type rifle was about 2.86. Wondering how they got 3″ shells in it is all.

    • Thinner magazine walls for more internal space maybe, or different feed angle. Would be interesting to see how it was put together or if 3in shells are actually less than expected length.

    • The length of a shot shell is measured before being crimped or after being shot in other words.So a loaded 3” shell is less than 3” long.

      • This^

        One of the advantages of brass hulls too, less length for the crimp required than any polymer hull = more contents in the hull. Theoretically anyway, all the brass hulls made in the current day are a bit cheaply made with respect to charge weights I’d feel comfortable with. Sans extensive comprehensive incremental destruction testing of which no one has done and which is inherently dangerous, not to mention expensive due to the costs of brass hulls et al.

        As an aside, it is truly a shame that with the proliferation of magazine fed shotguns, there is not one other company that has shouldered the mantle of producing better quality modern designed brass, composite, polymer, or whatever stronger material is applicable hulls that can withstand being loaded long term on a closed bolt without deformation issues.

  5. So…….If you have a REAL Armalite Ar-10 (trademark and copyright rights NOT infringed here), this thing WILL NOT work with your rifle. If you have a DPMS pattern 308 rifle (NOT an AR-10), then it will work. I’m sure Armalite will be interested to hear another company violating their trademark and copyrights…….

  6. This is such a cool idea – even though I realize anything AR-10 / DPMS 308 can vary for a good fit build (voice of experience). But the cost really took me back. I get this is a custom build upper, but dang!, for $1700-$2000 I can get a pretty decent tactical shotgun complete, a handgun to go with it, a bunch of accessories, and a couple hundred rounds of ammo. But now, Graham, you went and told me about this and I won’t be able to get it out of my head… As a gun builder, this is pretty trick and alot of stuff I build is just that. (My wife’s just glad I’m not into rebuilding cars like the neighbor.)

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