Sig Sauer Loses MPX “Muzzle Brake” Case to the ATF

Sig Sauer, known in the firearms legal for their willingness to litigate against the BATFE, has lost their MCX “muzzle brake” case in federal appellate court. The Appeals court agreed with the U.S. District Court of New Hampshire’s previous ruling that Sig’s MCX “extended monocore brake” was indeed a suppressor.

While US firearms enthusiasts may beman the ruling, it was hard for Sig to make an argument that the true intent of the extended brake was as a muzzle brake, not as a monocore for a suppressor (unlike that of the “arm brace”) . Per SeaCoastOnline:

In Tuesday’s decision, the appeals court agreed with the ATF that the disputed gun part is intended for use only in assembling or making a silencer because it was designed with features common to all silencers, but not muzzle brakes. The appeals court also agreed with the ATF that the part “is not of a type that requires, or is sold with, a muzzle brake” and that Sig had initially submitted a prototype of a gun, with the disputed part, that only functioned safely “if it was encased and thus had been made into a complete silencer.”

The appeals court noted that legal counsel for Sig Sauer agreed with New Hampshire’s federal court that Sig “had basically taken the cap off (of its) silencer… welded it onto the gun, and (was) just going to sell it as a muzzle brake.”

Basically, Sig had shown the court and the ATF that it was meant as a suppressor stack.

This is disappointing, as I believe Sig missed and opportunity and failed to use the case to compel the ATF to clarify what the definition of a suppressor truly is. Getting a solid legal definition would go a long way to clarifying the still murky waters surrounding “suppressors” including what comprises them, their parts, etc.

Nathan S

One of TFB’s resident Jarheads, Nathan now works within the firearms industry. A consecutive Marine rifle and pistol expert, he enjoys local 3-gun, NFA, gunsmithing, MSR’s, & high-speed gear. Nathan has traveled to over 30 countries working with US DoD & foreign MoDs.

The above post is my opinion and does not reflect the views of any company or organization.


  • Nicholas C

    That sucks

  • Budogunner

    Surprising no one.

    We’ve had a ton of pixies of what happens when you run a break under a rail, so the intent is clear.

    Don’t get me wrong, I love suppressors, but you can only bend the rules so far.

  • Retriever222

    I would just resubmit a similar version and say it is not supposed to be used in conjunction with a shroud.

  • Harry’s Holsters

    Does the “muzzle break” portion extend under the hand guard? If not then how is it dangerous?

    If so resubmits a new version with a inch “muzzle break.” That might actually get though. Plenty of shorter effect suppressors on the market.

    • J.T.

      Yes, The “muzzle break” portion extends under the hand guard. About a third of the way down if I recall correctly.

    • Duray


  • BattleshipGrey

    The Thought Crime Agency now has a basis for existence.

  • J.T.

    Sig has only themselves to blame. Their marketing people were pretty much using the fact it was a baffle stack as a selling point when it was first announce. It was only after that that the ATF took a second look at it and changed their minds.

    • wetcorps

      Yeah I remember the interviews at shot show, they were very obvious about it.

    • Younggun21

      I take a different position. If you doubt for one second that the ATF wouldn’t have seen what this was after it hit the market you are thinking far too much of the ATF. Some bureaucrat over there surely would have noticed where this was going and the precedent that was being set then you would have a bunch of people having to go get their firearms gunsmithed and have these nifty things ripped off of the end of their guns in order to comply with NFA regulations.

      Of course it is a selling point, it is an innovative feature that has never been tried before (to my knowledge) on a commercial firearm. And of course this was going to be taken to court by someone either an anti gun group or the ATF themselves. Could Sig have made some other convincing arguments in court? Sure. However I am also not a lawyer so I cannot opine on the proper legal course of action nor the ramifications should some of those arguments be found invalid.

    • Joseph Goins

      We can blame a lot on the gun culture hurting itself over, again. Everyone knew the shouldering the Sig brace wasn’t wrong. When everyone started doing it and marketing it as a “legal short barreled rifle without the tax stamp,” the ATF took a second look.

  • Bull

    So replace the stack with a permanently welded big tube…
    Should be harder to call it a suppressor

    • JumpIf NotZero

      The whole point was they wanted to sell the difficult to make part as a non-regulated item. Selling a tube is pointless, they are free to do that now.

      It would still leave people on the hook for making the more difficult parts, and then they would be all varied and different, some would perform poorly or with too much H backpressure, etc.

      SIG doesn’t want to see a bunch of Form1 MPXs, they wanted an NFA-hack and lost it for them and for everyone else in the future.

      • Younggun21

        It is hopefully not a total loss being a case study for how ridiculous and dated the suppressor legislation is. Technology and shooting sports have advanced so much that arbitrary government definitions on what is or is not a suppressor despite function and result, is simply antiquated. This will hopefully show the overreach of agencies like the ATF in their curtailing of technology that is good for the consumer and the public as hearing safe technology should be a priority for everyone.

  • nova3930

    Only suppressor in history that makes the gun louder….

  • Younggun21

    Why? This could have revolutionized the suppressor game making suppressors easier to acquire at far cheaper prices. You essentially would only have to pay for the tax stamp and the outer shroud that is far less expensive than a whole suppressor. I appreciate someone getting out there and fighting the good fight. I can’t personally spend all my time and money litigating against the feds so I am happy to support a company that will, especially one that is innovating and trying to get these products to the civilian market.

    • Mr Mxyzptlk

      “You essentially would only have to pay for the tax stamp and the outer
      shroud that is far less expensive than a whole suppressor.” How would that have been cheaper? You would still end up buying a baffle stack incorporated into the price of the gun, and would also then still have to buy the outer shroud. The only difference is that SIG would be charging people who didn’t turn it into a suppressor for a needlessly complicated component part, and if you did turn it into a suppressor then it would be integral to this one firearm and you couldn’t swap it onto others.

      • Younggun21

        It would be cheaper because the most complicated and expensive part that is being manufactured is coming with the gun which is not nearly as regulated as the suppressor. Suppressors as a technology aren’t anywhere close to as expensive as they are charged for. The regulation of the suppressors is what jacks the price up to a premium (just look overseas). In this way, the majority of the suppressor is already created and incorporated on to the rifle like any other muzzle device without having to go buy another entire unit.

        • JumpIf NotZero

          You have no idea what you’re talking about.

          Suppressors are here aren’t actually expensive because of regulation – but they are the best in the world here because of that regulation.

          Suppressors here aren’t expensive. There is a lot of research that has gone into them over the past 8-10 years. I PROMISE YOU that you can not build a single can on the market right now for less than it costs to just buy one.

          That research has gone into them because it’s a regulated market and it’s a PITA the work with. It’s an interesting position where the mfgs in order to make a product that’s worth jumping through the hoops for have gone the extra mile. You’re comparing European silencers, but those are pretty much worse in every regard and have only been getting better by copying US designs and practices.

      • Budogunner

        I guess the idea is you don’t have to pay a gunsmith to perm-attach the suppressor to ATF spec to make it a one stamp gun.

        Of course, with only one option for the core Sig could charge whatever they want and you’d never know the markup of the end-result suppressor. With muzzle cans you at least have market competition to keep things in check.

  • Younggun21

    Furthermore, the more of these cases we get out there and publicize (even if we lose) the more people see how absurd the laws on the books are. A person straddling the fence on gun rights could very well look at this ruling and come to the conclusion the laws surrounding the acquisition of suppressors has gotten so ridiculous and out of sync with the times that they support change in nearly century old legislation.

    • JumpIf NotZero

      Uh huh… And instead, we have the reality that ATF has decided more rules. Great. Just like we can’t replace suppressor tubes or transfer serial numbers.

      reality vs fantasy

  • Martin Grønsdal

    so this muzzle break is half way under the hand guard, and by accident, if you put a tube on it, it will function as a silencer?

    sounds like a smart business idea, given the legislation of silencers.

  • RickOAA .

    Oh, bummer.

  • guest

    Queue the crocodile tears from the “Pistol Brace, Pistol, Suppressor Stack and Bump-fire Society”.

    Come to think of it I think I reversed my opinion on these things. I say the more of it that exists – the better, because the sooner all the… should I say “gun law darwin candidates”… will entrap themselves and get themselves behind bars, as not to pollute anyone else with their ideas.

  • spasmonaut

    I believe the real case here is that suppressors should not be controlled, period. Then this nonsense would be a nonissue. Done.