Pistol-Caliber Carbine Cool

Parabellum Plus" The MAG AD9 Triples The Fun Factor Of Your AR

Nice camera catch! This mid-burst full auto shot caught the streak of the bullet leaving the muzzle.

The MAG AD9 is one of the most innovative AR15 accessories in a generation. It’s what every other pistol-caliber conversion aspires to be. And it doesn’t require any modifications to your lower receiver.

The MAG AD9 accepts reliable and inexpensive 9mm GLOCK magazines — which literally litter the landscape. Sporting inspired engineering and superlative execution, the MAG AD9 takes your favorite black rifle to some previously unexplored places.

Begin at the Beginning

Colt gave the pistol-caliber AR concept a whirl in 1982. The Colt Model 635 SMG used a modified M16 lower along with a custom upper that eschewed a forward assist in favor of a big rubber ejection buffer. The gun ran 9mm Parabellum from the unlocked blowback and was both handy and reliable.

The rub? The conversion required drilling holes in the lower receiver. That’s not a big deal when the receiver began as a $40 homebuilt project. When it is a $14,000 transferable machinegun chassis it becomes another matter entirely! Additionally, the Colt gun used proprietary 32-round double-column box magazines adapted from those of the Uzi. It turns out there was a way to do things better.

Compact, carryable, hi-cap and very handy: The MAG AD9 turns your favorite
black rifle lower receiver into something altogether different.


The beating heart of the MAG AD9 system is a drop-in magazine well conversion device that slides into place within the magwell of favorite ARs and adapts it to accept polymer GLOCK magazines. The adapter sports its own separate magazine release that’s easily accessible on the right rear aspect of the magwell. The unit is secured in place via the host gun’s magazine catch.

The MAG AD9’s set screws excise any wiggle that might result from variations in tolerances among the many millions of AR receivers out there. One component is called a gib and snugs the unit fore and aft. Two nylon-tipped Allen screws put tension against the magazine catch and keep everything tight.

The MAG AD9 device has a built-in ejector and — literally — couldn’t be any easier to manage. Installation takes maybe 3 minutes and removal about the same. Once installed the adapter accepts mags of any capacity. Standard pistol magazines are nearly flush with the bottom of the magwell while 33-round boxes give you plenty of on-tap chaos without undue bulk. Compact drums multiply the fun.

The Stern Defense upper receiver assembly provided for this project sports a short pistol-length barrel and a heavy-weighted bolt carrier cut for full auto operation if required. Everything was exquisitely well executed and nicely finished.

Load up! The MAG AD9 magwell adaptor lets you convert your existing 5.56mm
AR lower to 9mm using either Glock or S&W M&P magazines.

Full Auto Enhancement

If you’re blessed with a transferable full-auto M16 lower, the MAG AD9 device is fun on an apocalyptic scale. I’m old, so I bought mine for $600 back in 1987 and thought I was getting ripped off at the time. If only I could go back in time, hock my house, and put all that cash into transferable M16 receivers I’d be a wealthy man today. Should you be deprived of such a nifty bauble, fret not. There are some truly epic options for the MAG AD9 available even today.

A rifle lower receiver can sport any manner of buttstock but will demand a 16-inch barrel for legality. Pistol lowers open the door for tubes of any length but cannot include a stock. None of this has any bearing on crime or criminals. It is simply a ridiculous administrative burden we law-abiding gun nerds must bear.

A few years ago a guy named Alex Bosco thought up the Pistol Stabilizing Brace (PSB) as a tool to allow injured veterans to continue enjoying the shooting sports. The BATF ruled that installation of such a brace did not threaten the pistol classification of the host weapon, and a whole new family of firearms resulted. As a result, a Stern Defense 9mm upper, MAG AD9 magwell adapter, AR15 pistol lower, and an SB Tactical PSB can synergistically combine to form something greater than the sum of its parts.

Like a whole new gun: The MAG AD9 9mm adapter installed on this transferable
machine gun lower is easy to install and requires no modifications to your lower.

Tweaking The Beast

Reciprocating firearm actions are remarkably elegant little machines. An autoloading firearm takes a bit of chemical energy pent up within a cartridge and then uses it to both throw the bullet downrange and reload the gun of its own accord. The resulting mechanical synchrony is a beautiful thing to behold.

The bolt carrier on the Stern Defense upper is weighted to prevent bolt bounce — a vile event occuring when your unlocked bolt chambers a round and then recoils a smidge before fully seating again. If the hammer falls in that pregnant moment then reliability suffers. It’s a bigger deal on machineguns than semi autos but it still plays a part regardless. Heavy buffers eliminate bolt bounce and cause a slower rate of fire in full-auto guns.

With a really heavy buffer in place, the 9mm round lacks the horsepower to fully cycle the action. I swapped out to a lightweight buffer I had in my parts bin, and the thing ran like a scalded ape. Lightweight buffers will already be found in the vast majority of semi-auto black rifles in circulation, but should you end up with a MAG AD9 and find it doesn’t always recock the hammer, then the buffer is your problem. It’s no deal-breaker. A lightweight 3-oz. buffer will set you back 10 bucks online.

It’s Alive!

The MAG AD9 opens new ballistic vistas. Recoil is all but nonexistent, ammo is cheap, and the resulting gun runs quickly, accurately, and well. The support thumb finds the magazine release easily, so mag changes are as fast. The bolt locks back on the last round fired just as it should. The trivial adjustments to the manual of arms demanded by the magwell adapter are mastered in moments. Once I installed a lightweight buffer I couldn’t make it choke, no matter the host chassis.

With a solar-powered red dot sight from Holosun perched atop the flattop upper the resulting package is a formidable close combat tool small enough to tuck into the center console of a proper pickup truck , and accurate out to as far as you ever might reasonably shoot in a defensive situation. Choose your defensive handgun carefully, and both guns can run from the same magazines.

The heart of the MAG AD9 conversion is the magazine well adapter. This device pops in
place painlessly and accepts unmodified GLOCK mags. The last-round bolt hold-open feature
is seamlessly retained. Cartridge commonality is good. Magazine commonality is great.


Taking it up a notch, you could finish out an 80 percent receiver at home, outfit the resulting gun with a PSB, and leave the short barrel in place. Such a gun can be legally crafted using some basic shop skills. With Gene Stoner’s remarkable black gun as a foundation it is breathtaking to ponder how far we’ve come. With a remarkable MAG AD9 magwell adapter, running 9mm through your favorite black rifle is a fresh new experience, and ammo is cheaper to boot.

Stern Defense
Ph: (417) 932-1128
[email protected]

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