Sig Sauer MPX

Will Upgrading the Sig Sauer MPX to an AR-15 Trigger Void Your Warranty? 13

UPDATE: When this article was first published in June 2015, SIG Sauer’s official position was that installing an aftermarket trigger in a SIG MPX would void the warranty. They have since changed that position. Aftermarket triggers no longer void the warranty, though SIG won’t be responsible for any damage to aftermarket triggers (which makes sense). Read the MPX FAQ article on (I also publish that blog) for the most up-to-date info. The article below is still informative as to how the stock MPX trigger works and the importance of the trigger bridge, as well as why most aftermarket triggers will not fit in the MPX with the trigger bridge.

I’ve been loving my new Sig Sauer MPX. It can legally be a pistol, carbine, or SBR… depending on its configuration (and how much tax you’ve paid). One of the selling points of the weapon is that its primary controls borrow heavily from the ubiquitous AR-15, meaning anyone who knows how to operate an AR knows how to operate an MPX. In fact, Sig was originally quick to point out that owners can drop in any Mil-Spec AR-15 trigger group and the gun will operate.

But apparently that could now void your warranty.

The primary issue revolves around an element in the MPX’s trigger group called the Trigger Bridge. This was originally brought to my attention yesterday through an email discussion with the fine folks at Osage County Guns, who received the following alert from their Sig Sauer rep:

The SIG MPX has a fire control group specifically designed to accommodate the high impulse of the MPX operating system. The MPX Trigger Bridge (indicated below by arrow) is in place to reduce wear and extend the life of the factory installed fire control components. Removal of the Trigger Bridge is NOT recommended and will void the warranty of the firearm. While the SIG MPX fire control components are similar to the AR15 platform, and aftermarket fire control components may fit, installation is NOT recommended in the SIG MPX. Doing so may result in damage to the MPX proprietary fire control components or the aftermarket components you installed.

The rep also included a rendering of the MPX trigger group showing the location of the Trigger Bridge (which I edited a bit for inclusion in a YouTube video on this subject):

Sig Sauer MPX Trigger Bridge

Sig Sauer MPX Trigger Bridge

In my limited time with the gun, I am seeing very slight wear marks on the Trigger Bridge where the MPX’s hammer makes impact upon its return. Whether or not this piece is truly needed seems to be the topic of some debate in online Sig circles, but what gets me leaning Sig’s direction is the fact that they purposely included this “extra” part in the gun, and extra parts cost money. If a company is willing to spend extra money on a component, I have to believe that their test data justifies the expense (I’m doing some more digging, and am trying to get in touch with an engineer at Sig that can speak to that).

Watch this short video where I discuss installing an ALG QMS trigger in the Sig Sauer MPX, and show why it’s not as simple as merely installing the Trigger Bridge with an aftermarket trigger group:

So for now, it seems we might be stuck with the stock MPX trigger. But it wouldn’t surprise me if Bill Geissele was already locked away in his workshop, tinkering away at a solution that will keep both Sig and gun modders happy. Are you listening, Bill? 🙂

Thanks again to John Dawson and the rest of the crew at Osage County Guns for originally bringing this to my attention. As always, I welcome your questions, comments, and feedback below. And I’d love to know whether you’d choose to keep the stock 9lb+ (!) trigger, or go ahead and void your warranty with an aftermarket trigger group.

  • darith

    The trigger bridge is interesting. I would like to see the fitment of other higher-end triggers for the MPX.

    The easiest and cheapest solution to decrease the trigger pull weight is replacing only the trigger spring with a JP Enterprise reduced power hammer spring from their reduced power spring kit (it will only run you $10). This will also allow you to use your original trigger group components.

    Never replace the hammer spring with the JP Enterprise reduced power hammer spring as it can result in light primer strikes on steel-cased ammo (only recommended on JP Enterprise trigger group parts).

    • Hi, Darith. That’s great advice. We’ve been talking about the JP trigger spring (and you’re right… NOT the hammer spring for the light strike issue) over on I’m going to try the ALG spring first, just to see if there’s any difference. If not, the JP is likely my next option. Thanks!

  • Bill Geissele

    HK guns also have the bridge. High impulse guns will toss the hammer back violently so that the hammer will hit the disconnector. By repeatedly striking the disconnector it will actually fracture over time. As a trigger similar to the Sig the ALG trigger may have this problem. However, the Geissele triggers are designed so that the bridge is not needed and the hi velocity of the bolt carrier will not effect it. And even if there was a problem Geissele’s golden warrenty is always there to help you out.

    • Hi, Bill. Thanks for stopping by and sharing your insights. It comes as no surprise to me that your triggers are engineered tough enough to be immune from the problem. And always good to be reminded that even in the unlikely event that one wasn’t, a replacement is just a phone call away. 🙂

    • Steven Cox

      Awesome Bill. I already have two of your outstanding SSA-E’s in two of my AR’s. Would you recommend the SSA-E for the MPX?

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  • lionsfan54

    So, would a drop in trigger skip all this problem?

    • You can run a trigger that doesn’t require the bridge (such as a Geissele, Timney, etc.) and then you’ll have to rely on the trigger maker’s warranty if there are trigger issues. Geissele has said their trigger will work in the MPX, and that they will stand behind it.

      • lionsfan54

        Yeah, saw a Timney get reviewed and it had a FTF after about 50 rds. They sent it back and got “extra lock tite” put on it and it worked again. Not sure how comfortable I am having my home defense setup hoping that the the “extra lock tite” would hold up when I really need it.

        Probably will go with the JP trigger spring fix and then have my guy work on the existing trigger parts to smooth then down, etc.

        Thank you very much for this piece of writing!

  • Save

    I just got the gen 2 sig Mpx. I tried a RRA 2 stage trigger and the bolt wouldn’t slide closed with the magazine in place. So I returned it and installed the Gisselle trigger ssa-e and had the same problem. Anyone else having issues? Any tips?

  • Rob Miller

    Aside from the warranty, why couldn’t you grind down the after market disconnector to the same geometry as the Sig trigger and use the bridge?

    Is the bridge acting as a buffer to protect the disconnected, similar to how the buffer in a 10/22 protects the receiver? Why not attach a piece of lexan on the top of the connector arm to buffer the blow?

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  • DARoberts

    I installed a S3G trigger in my MPX. Seems to work fine. The bridge in the MPX looks like it’s purpose is to protect the disconnector from that large spine on the stock hammer. The S3G’s hammer does not have that spine and doesn’t appear to pose any threat to the disconnector. Took it to the range and put about 25 rounds through it. Dissassembled it and examined for evidence of hammer strike on disconnector – found none. I don’t see any problem with the S3G trigger in the MPX.