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 RealGunReviews.com (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

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.