SIG MPX Gen 1 vs Gen 2 FAQ Video Analysis

To address some of the confusion concerning the differences between the SIG MPX Gen 1 and Gen 2, SIG Sauer released a video yesterday (embedded below) in a “frequently asked questions” format, presented by SIG Sauer employee Adam Johnson. The video doesn’t list his official title, but Adam’s LinkedIn profile says he’s a formerly active Marine (there’s no such thing as an “ex-Marine”) who’s been an “Assistant Product Manager – Rifles” at SIG Sauer since September 2014.

Here’s the video, and then this article will analyze the questions and answers that were addressed:

Is it true that there’s an MPX Gen 2?

SIG’s answer is that the MPX Gen 1 is a 9mm platform, and that the Gen 2 has been “updated to accommodate the multi-caliber functionality of the platform.”

That’s a vastly different story than early purchasers of the MPX Gen 1 (including myself) were led to believe. In fact, on my Gen 1 MPX lower, it’s clearly stamped “MULTI-CAL” where the caliber is normally stamped. In the August 2013 MPX brochure (still downloadable from SIG’s website) the first bullet point on the first page of the brochure says “Easily convertible to 9mm NATO, .357SIG or .40S&W.”

As of today, the “Specification” tab on the MPX page of SIG’s website still shows all three calibers (with no mention of different generations):

Screen shot of SIG's MPX website page "Specifications" tab

Screen shot of SIG’s MPX website page “Specifications” tab

What does this mean for Gen 1 owners who want to do a caliber exchange kit?

For Gen 1 customers who want to run a caliber other than 9mm, SIG will offer them a “complete Gen 2 upper receiver assembly at the same cost as a caliber exchange kit” — but only in .40 or .357 SIG. Gen 2 owners wanting to do caliber changes can simply order a caliber exchange kit.

This left me wondering what Gen 1 owners with 8″ barrels will have to do if they want to run a different 9mm barrel length… such as the 4.5″ barrel that SIG has announced is on the way.

At this point of the video, it sounded like they’d have to purchase either a .40 or .357 SIG Gen 2 upper, then also purchase a Gen 2-compatible 9mm barrel in the desired length. Later on in the video, however, Adam reassures Gen 1 owners that they will eventually be able to purchase 9mm Gen 1 barrels in various lengths… though I’d be lying if I said I was convinced at this point.

What I hope SIG does is decide to allow Gen 1 MPX owners to purchase a 9mm Gen 2 upper with any size barrel they want, for the price of the caliber exchange kit. Let’s hope that by the time all these parts are actually available for shipping, SIG rethinks their approach to Gen 1 owners… who are currently feeling a bit like beta testers.

What if I want all 3 calibers for the SIG MPX GEN 1?

The answer’s basically what I alluded to above. Once you’ve purchased a Gen 2 upper, it will work with any Gen 2 “cal-ex” (caliber exchange) kit you want to purchase.

As a SIG MPX Gen 1 owner, will I be able to change barrel length in 9mm?

This is the question I alluded to above. And I really hate to call SIG out on this, but there’s a “pregnant pause” at the 2:14 mark of the video before Adam answers this particular question. Maybe that pause means nothing, and my spider sense as an ex-product manager myself is tingling for no reason.

Adam attempts to reassure Gen 1 MPX owners that “SIG Sauer will continue to support the MPX Gen 1, by offering different barrel kits in either 4.5, 6.5, or 8 inch… in 9mm only.” I wish I could say I felt reassured, but I have to admit that based on what’s happened with the MPX so far, until I have a 4.5″ OEM barrel (along with the matching hand-guard) in my hot little hands, I won’t be convinced.

Will my Gen 1 9mm magazines fit and function with a Gen 2 9mm Upper Receiver, and vice versa?

The short answer from SIG is “yes, they will fit and function in both Gen 1 and Gen 2.” But the longer answer is “for optimal reliability we definitely recommend using like-generation magazines with matching generation MPXs.” Adam assures us SIG will continue to supply both Gen 1 and Gen 2 magazines, with Gen 1 magazines (like the Gen 1 MPX) being 9mm only.

SIG changed the color of the magazine followers on the Gen 2 magazine for easy identification between the two versions. If the magazine has a black follower, it’s a Gen 1 (and therefore 9mm). If it’s flat dark earth (FDE) colored, it’s a Gen 2.

Will my Gen 1 Lower Receiver fit and function with a Gen 2 Upper Receiver?

The answer is “yes,” but this question was answered earlier in the video, since providing a Gen 2 upper to Gen 1 owners is SIG’s solution to providing multi-cal capability to Gen 1 owners.

This isn’t mentioned in the video, but the good news here is that the serial-numbered portion of the firearm is the lower receiver, so if you’ve SBR’d your Gen 1 MPX (like I have), you can swap in any Gen 2 upper (provided the barrel length is appropriate for your NFA paperwork) and be good-to-go.

Will a Gen 2 MPX handguard fit on a Gen 1 MPX?

This question is important because the Gen 2 MPX ships with a KeyMod handguard, while the Gen 1 has a SIG-proprietary “add a rail” screw pattern, making the Gen 2 handguard more desirable for those looking to kit it out. The good news is that the Gen 2 and Gen 1 handguards are interchangeable, and Adam demonstrates this at the 3:46 mark of the video.

How do I know if my SIG MPX is Gen 1 or Gen 2?

On the Gen 1 MPX, the top rail is a standard 1913 Picatinny rail. On the Gen 2, the rail on the upper (as well as the handguard) is “scalloped” to match the look of the SIG MCX. Gen 2 MPXs also ship with a KeyMod handguard.

Immediately, this makes me wonder whether SIG plans to make KeyMod handguards available without the scalloped rail to match the look of the standard Picatinny rail system on the MPX Gen 1. My gut tells me “probably not.” This is another reason why I hope SIG wises up and allows Gen 1 owners the choice to purchase 9mm uppers (with the scalloped KeyMod handguard) in addition to either the .40 or .357 SIG.

Are either guns compatible with aftermarket trigger groups?

This question was especially interesting to me, since I’d written a post on my personal blog (and posted a video on my personal YouTube channel) on June 10, 2015, quoting an alert from a SIG Sauer factory rep that stated:

“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.”

Since writing that article and creating that video, I’d chosen to ignore the rep’s warning and have been happily running a Geissele SSA in my Gen 1 MPX. I felt comfortable doing so because Bill Geissele commented on that article, saying:

“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 warranty is always there to help you out.”

So when Adam brought up this question at the 4:44 mark of the video, my ears perked up… particularly since he mentions it’s the most common question SIG receives about the MPX.

SIG’s current position on aftermarket triggers is different than originally stated in the SIG rep’s alert. SIG now says that the short answer is “yes, and aftermarket trigger assembly will fit and it will function into (sic) the MPX Gen 1 or Gen 2. The trigger hole locations are the same as a standard AR-15. The trigger will fit and function as it would in any other AR-15 platform.” So far, that’s nothing new. I demonstrated as much in my MPX trigger video. Adam does address the “higher impulse of the inner workings of the MPX,” which is the justification for the trigger bridge included with the stock MPX trigger (which doesn’t seem to work with any of the aftermarket triggers I’ve tested in mine).

The 5:19 mark is where things get interesting. Adam says “putting an aftermarket trigger assembly into an MPX is not going to damage the MPX itself, and it’s not going to void the warranty of the MPX.”

Yep, you heard that right. SIG’s position (as it should be) is that aftermarket triggers do not void the warranty on an MPX. Adam goes on to explain that “it may cause damage to the aftermarket trigger assembly, and that SIG Sauer cannot guarantee.” That seems perfectly reasonable to me, and is a welcome change from their original position. I’ll keep running my Geissele SSA, but now I’ll also admit it publicly. 🙂

Where can I purchase supporting accessories for my Gen 1 or Gen 2?

I’m gonna call a little bit of BS here. I don’t believe this is actually a “frequently” asked question for SIG Customer Service. This feels more like an “exit through the gift shop” kind of moment in the video, especially considering it’s the final question.

As you might have guessed, the answer is simple: shop online at or call customer service! 🙂

Adam includes the disclaimer that MPX accessories are “high demand items and they may be back-ordered.” That’s what’s called “an understatement,” folks.

What Wasn’t in the Video…

Conspicuously absent from this video was any discussion of the MPX charging handle issue, where owners (including myself) have noticed that closing the bolt while the charging handle is pulled back results in cosmetic wear to the lower (not such a big deal) and what I consider substantial wear and tear on the upper.

In their defense, SIG did send me what they called “upgraded” charging handle to address the issue, but my calipers and I weren’t able to find any difference whatsoever vs. the original handle, and the “newer” handle had no effect on the problem. I now manually “click” the charging handle before releasing the bolt… which makes me feel much less cool than were I able to merely yank it and slap it (that sounds dirtier than it actually is).

Final Thoughts

All tongues in cheeks and skeptical criticism aside, I congratulate and thank Adam and SIG Sauer for releasing this video (I’d also like to personally thank Adam for his military service). From charging handle-issues, to paperclip fixes, to lack of Gen 1 multi-cal, to back-order issues with telescoping and folding stocks, to confusion with the Gen 2… the MPX’s first full year of existence hasn’t exactly been “smooth.” But all that aside, it’s still one of my favorite firearms in my collection. In fact, an SBR’d and SilencerCo suppressed Gen 1 MPX with a Streamlight and an Aimpoint PRO is my “nightstand gun.” I sleep better knowing it’s handy, and it always brings a smile to my face at the range.

I’m still a bit skeptical about just how much “support” SIG will provide Gen 1 owners, and for how long. I hope I’m proven wrong.

But I’m also encouraged by SIG’s position change on aftermarket triggers, and have to believe that they’ll eventually match supply with demand for aftermarket parts.

What are your thoughts… on this video, SIG Sauer, and/or the MPX in general? Let me know in the comments below.

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