Stern Defense Caliber Conversion for ARs: New Model Uses P320 and M&P Magazines

Stern P320/M&P magazine adapter with a Stern 9mm upper and Nikon P-Tactical SPUR Reflex Sight.

Stern Defense makes great AR magazine-well adapters that turn your AR lower into a pistol caliber carbine.  Their latest version, the MAG-ADMP9&40/P320, uses both SIG P320 and S&W M&P magazines.

With the gun ban scares over the last few years and the popularity of 80 percent lowers rising, most of you reading this have AR lowers in a closet that you never built. Here is the excuse to build the pistol caliber AR you always wanted. If you only have one AR, in one minute, you can change the upper, buffer and add a magazine-well adapter. Boom, new gun!

Stern MAG-AD adapters are very easy to install. They lock in hard with no wiggle after adjustment. They hold your mags in and drop free when the release-control is engaged. Unlike some other PCCs, the bolt holds open on the last round every time. MSRP $186.99


The newest model runs with P320 and M&P magazines in 9mm or .40 S&W. I have tried these in several receivers with multiple uppers. They all ran well with any combination of magazines, uppers and lowers.

Shooting with the Stern P320/M&P adapter using a .40 S&W P320 magazine and .40 caliber Lone Wolf upper with a Holosun red dot sight.


Pistol caliber ARs are cheap plinkers and great training tools for new shooters. You can safely shoot handgun steel and use the magazines you already have. Law Enforcement agencies can build an AR that accepts their duty pistol magazines for the ultimate police patrol rifle.

Because there is no MILSPEC for pistol caliber ARs, each manufacturer makes them a little differently. If you buy everything from the same place, your chances of an easy build like mine are much higher. Maybe you will be lucky. However you decide to get there, now you don’t have to build a dedicated lower. Stern has a web page you can check before purchasing to ensure compatibility with your setup so you don’t buy parts that don’t fit together.

Stern also make a nifty extended polymer magwell that clamps on and speeds up reloads.

I have worked several AR conversion systems, but nothing like the Stern Defense system. I got their MAG-AD9 conversion kit, with adapter, complete 9mm upper and their 11-ounce carbine buffer. These adapters require the use of a 9mm or .40 caliber barrel, and a Glock cut bolt. You can buy the bolts and barrels from Stern separately.

If you have a registered full auto lower, Stern gives you caliber options. Short Barreled Rifle? Why not blast pistol ammunition? If you have a 9mm suppressor, there is no better host than a 9mm AR. These conversions provide more fun for your training dollar.

Technical Information:

    • Made in the U.S.A.
    • Constructed from 6061 T6 aircraft grade aluminum.
    • 4140 NiB coated feed ramp.
    • Hard coat anodized type 3.
    • Sturdy Construction.
    • Ergonomic mag release.
    • Works with any mil spec ar15 lower receiver.
    • All lower components besides the adapter remain stock

Visit SternDefense to learn more.  

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About the author: Mark Miller is a former Customs Agent and a Green Beret who served in Afghanistan and a number of other live fire locations. A student of firearms and shooting, he is an FFL and a SOT. The guiding philosophy of his life is that terrain and situation dictate tactics and the enemy always gets a vote on any plan.

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