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Flux Defense’s MP17 Turns Your SIG Into a Compact PDW

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Pistol-caliber carbines have experienced a huge resurgence over the last couple of years. But really, what’s the point of a rifle-sized weapon that doesn’t have the inherent capabilities of a rifle? Yeah, we think so too. But something slightly larger than a pistol that gives increased range and accuracy we absolutely welcome. This is the concept of the Flux MP17 conversion. At the 2019 SHOT Show, Flux Defense made quite a splash with their automatically deploying Glock pistol brace (see RECOIL Issue 44). Fast-forward almost a year, and we now have a second generation made exclusively for the Sig Sauer P320/M17. The time spent in development really shows.

The Flux MP17 has both pistol “brace” and short-barreled rifle options, but we opted for the SBR version for a couple of reasons. First and foremost, we’ve learned from internal memos that the Bureau of Alcohol, Tobacco, Firearms and Explosives and the U.S. Attorney’s office may pursue charges for illegal SBR manufacture if the host weapon weighs less than 36 ounces unloaded. Since a full-size P320 only tips the scales at just over 29 ounces, we decided to simply register it to avoid any potential issues. For those following along at home, an e-filed Form 1 takes mere days instead of months, so better safe than sorry.

Right out of the box the MP17 has the ability to accept dozens of optics.

To that end, the Flux MP17 grip itself features a handy cutout on the left side to engrave your manufacturer information directly on the chassis. However, that isn’t your only option. The law states that manufacturer information can be engraved on the receiver, frame, or barrel — though the chassis itself is the easy button in this instance. If you plan on moving one registered chassis between multiple hosts, carving that cutout yourself on other grip assemblies will make for an easy day.

While there’s little doubt that the Flux MP17 conversion is meant to be used with a full-size M17 or P320 variant, you can actually use it with a compact slide and barrel, with some caveats. You’ll either need to install a pistol compensator to direct the muzzle blast to the top and sides or run a silencer. You’re somewhat limited on the suppressors that can be used with the MP17 in this configuration — 1.375 inches or less in diameter — but most of the common ones will be A-OK.


The “wow” factor of the original Glock Flux brace was the fact that it could be automatically deployed with a simple push of a button on the right side of the unit. And indeed, that particular bit of flashiness is retained with the MP17 conversion. If the deployment of the stock is impeded, all you have to do is grab it and physically extend it until it locks in place. Easy peasy.

Unlike the original Flux Brace, the MP17 SBR conversion features a longer length of pull, making it easier to shoulder. There’s also some texturing on the back of the SBR stock to help ensure it stays in place. Folks with longer arms than our 5-foot-10 author may still find themselves scrunched up a little, though.

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Integral to the Flux MP17 conversion is a holder for a spare magazine. Simply drop your spent magazine and give a swift tug on the forward spare for a quick swap. With two 21-round magazines, this gives you 42 rounds on tap without reaching for that spare on your belt.

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