12-Gauge Shortgun: Mossberg’s Latest Dumpster Defender Transformed Into A Versatile SBS [BUILD]

Photos by Kenda Lenseigne

The most appealing aspect of shotguns is their versatility. No, you’re not going to connect at a mile with a shotgun, but inside 100 yards, they can do a creditable job of putting holes in stuff that requires ventilation; everything from humans (buckshot), deer (slugs), and small game (birdshot). 

Things that make small arms more versatile and useful include short barrels, folding stocks, and semi-auto functionality, and if you don’t believe that, try working from inside vehicles or clearing a room with a PRS rifle. So how about we combine the two?

Mossberg apparently can’t keep up with demand for their recently introduced 990 Aftershock “firearm,” so if you happen to come across one, you may want to snag it. Chances are, if you wind up with buyer’s remorse, someone else will take it off your hands pronto. A 14.75-inch barreled semi-auto 12-gauge seemed like a perfect place to start putting together a do-everything shotgun, so we got right to it.

BASE GUN: MOSSBERG 990 AFTERSHOCK

• Caliber: 12 gauge
• Capacity: 5+1+1
• Barrel Length: 14.75 inches
• Overall Length: 24 (stock folded)
• Weight: 7.4 pounds (as shown)
• MSRP: $1,120

The 990 Aftershock shares much of its architecture with the 940 Pro, which has proven itself in the years since its introduction in 2021 and is a much better gun than its predecessor, the 930, which was hot garbage. Users can expect a nickel-boron-plated gas system, which should be good for at least 1,000 rounds before cleaning, as well as oversized controls, a high-vis magazine follower, and beveled loading port. 

The gas system is, to a great extent, self-adjusting to accommodate the wide variety of shells you can expect to cram into the mag tube; everything from standard velocity, 1.25-ounce birdshot to 3-inch magnum slugs. It does this by delivering a large amount of gas to the piston, then venting the excess forward via a heavily spring-loaded valve. 

Due to the barrel’s comparatively short dwell time, we found it to be a bit fussier than its longer siblings, but if you stick to ammo in the midrange of what can be found on the shelves, it ticks along nicely. We had issues with some 3-inch magnum turkey loads with heavy payloads, but S&B buckshot and slugs, as well as Hornady American Gunner loads were gobbled up like candy. 

The rest of the internals are fairly vanilla. Bolt lockup is achieved via a single lug that’s cammed into engagement with a cutout in a barrel extension, and there’s a single extractor and fixed ejector to get empties out of the gun. 

Unlike the 940, the 990 doesn’t have a recoil spring located in the butt, driven by a rat-tail extension on the rear of the bolt carrier. Instead, its spring is wound around the magazine tube, tucked under the forend. This allows for the installation of a rear grip or, in this case, a Picatinny rail adapter and folding stock. 

We should probably pause at this point for the usual disclaimer about NFA items, the need to have an approved Form 1 in hand before installing anything that would otherwise create an illegal short-barreled shotgun, and how the requirement is an unjust infringement on an enumerated right. 

If this was an SBR built on an AR-15 receiver, we know some who’d be tempted forego the tax stamp, as the ATF screwed the pooch with its asinine overreach regarding braces, rendering short-barreled rifle restrictions largely unenforceable. But this is a much rarer beast, so please fork over your 200 bucks and do it all legal-like. 

Once squared away, we sourced a pistol grip adapter from KE Arms. The rear of the 990’s receiver is identical to that of 500 series pump guns, so we figured this piece would bolt straight up and allow us to add the desired features. Nope. While the receiver is the same, its trigger guard is larger, so we wound up having to do some major surgery to get everything to fit. 

Following the sacred tenets of shade-tree gun plumbing, we ground on the least expensive part, and once enough metal had been removed the adapter was installed, along with an AR-15 pistol grip. 

Not content with that, KE Arms also sent over one of their red-dot mounts, so an ACRO footprint RDS from C&H Precision was added to the mix, along with a folding stock from Samson Mfg. We’ve used this model on a few different builds. It’s solid enough to handle 12-gauge recoil and also give plenty of adjustment options in order to line up the shooter’s eye behind the RDS.

Any defensive long-gun should have a light and sling, and fortunately Mossberg recognizes this, equipping the 990 with a polymer sleeve, the main function of which is to secure the barrel, but also is blessed with M-LOK slots on the sides and bottom. We used them to mount a SureFire Mini Scout light and QD sling swivel cup. 

ROUNDS DOWNRANGE

As this build was supposed to validate the versatility of short shotguns, we assembled a wide range of ammo and took the gun to a bunch of different locations. Beginning on our local range, we zeroed the red dot with slugs and patterned the barrel with a variety of birdshot and buckshot loads from as many different manufacturers as possible. That done, we reached for the box of random shotshells, which seems to accumulate after various 3-gun matches and truck cleanings, and tested reliability. 

After it went through those without a hitch, it was fed ammo from the extreme ends of the spectrum. As expected, Federal Metro Field loads, which are loaded to achieve a muzzle velocity of just 900 feet per second, did not cycle the action. We experienced failures to eject with Hevi Shot Magnum Blend 3-inch turkey loads, which are so unpleasant to shoot that we didn’t feel like trying to dig any deeper as to the cause. And there were a couple of FTE’s with Hornady Critical Defense 8-pellet buckshot, which is pretty spicy at an advertised 1,600-fps muzzle velocity. 

Following that first range session, which totaled around 120 rounds, an old buddy popped up on the radar, in town to visit clients and get some Arizona spring sunshine. He suggested we go bust clays, so the little Mossy got to rub shoulders with Perazzi and Beretta over and unders, much to everyone’s amusement. After the initial side-eye, everyone on the skeet field wanted to put some shells through it, and the cylinder-bored barrel had no problem hanging with the big dogs.

Due to advances in shotshell technologies, specifically the adoption of tungsten shot and Flite Control wads, it’s no longer uncommon to hunt turkeys with smaller bores. Figuring that dropping a tom with a shorty shotty would be eminently doable, the 990 was thrown first into a Pelican case and then into the hold of the Greyhound of the skies. 

Emerging on the family farm of Doug Duren in Wisconsin’s Driftless area, we spent a pleasant couple of days in good company chasing turkeys around the woods. During that time, we were as successful as guys carrying 26-inch barreled dedicated turkey guns. Which is to say, the turkeys won. But the advantages of a short barrel and folding stock were readily apparent when it came to riding around in ATVs and sitting in ground blinds.

On arriving home, we took the gun back out for several more range sessions, where we ran heavy relays of buck and slugs, on steel and got the barrel hot enough to burn flesh when we quad-loaded it. 

Like the 940 it’s based on, a few minutes with a Dremel tool to knock off sharp edges in the loading port area pays dividends when stuffing shells, and it can be ghost-loaded to boost capacity to seven rounds total, including one in the chamber and one on the lifter. 

There are a couple of areas where improvements could be made. First, there’s no provision for choke tubes. In order to wring out maximum performance from buckshot loads in defense of home and hearth, this needs to happen, and given the existence of multi-choke guns in the rest of Mossberg’s lineup, there’s really no excuse. 

Secondly, while the gun’s safety location works great with a conventional stock (or when using the donkey dong grip the 990 Shockwave ships with), it sucks big time when you add a pistol grip. Mossberg recently introduced AR-style safeties on some of its offerings, and this would be a natural place for that feature to land. 

The 990 Shockwave is a good place to start for a versatile SBS build. It’s not perfect, but if you can live with its shortcomings (or address them — this one’s headed to a ’smith to be threaded for choke tubes), then it’s probably the best platform on the market right now. As they’re selling every one they can make, don’t expect Mossberg to bring out the choked, rotary safety version anytime soon, but when they do, it’ll check every box we have. 

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