I have had a Gen I MPX for years, and recently picked up a Gen III (although Sig only acknowledges two different generations). After comparing the two, I noticed that there were LOTS of differences and wanted to give a comparison. In all pictures, the Gen I is on the top or on the left, and the Gen III is on the bottom or right. Also, my Gen I already had a series of aftermarket parts installed so some of the OEM parts weren’t available to be pictured. Most noticeable is the Gen I Handguard which I ditched long ago and replaced with a keymod Gen II handguard.

Differences are as follows (picture album:https://imgur.com/a/jxKnKbQ )

-Handguards: as previously mentioned, Gen I had the Sig add a rail system (not pictured). Gen III is MLOK.

-Pistol Grips: Gen I had a bulbous grip also found on the MCX (not pictured). Gen III has a grip that feels very similar to a Magpul MOE but with more texturing.

-Ejection port: Gen III is larger.

-Reinforcing plates on left of receiver: The Gen Is formed an inverted L. Gen IIs formed a T. Gen III completely did away with the full size plates and now has reinforcing screws.

-Rear of lower receiver: the Gen III has a cutout in the top of the rear portion to allow for the recoil spring retaining plate to nestle into it.

-Iron sights: the screw that attaches the iron sights was changed from an Allen screw to a hex head.

-Charging handles: the Gen I was a non ambi, very AR-esque design. Gen III is an ambi with nice larger latches.

-Barrel clamp: Gen I has bolts that thread into separate nuts. Gen III has a single piece that both bolts thread into.

-Gas ports/gas piston: Gen I has a small circular gas port on either side. Gen III has an oval shaped gas port on either side. The piston system is redesigned as well.

-BCG: Design between the two was drastically changed. The firing pin retaining pin was moved from the left on Gen I to right on Gen III. The piece that the charging handle catches on the top of the BCG was redesigned (less steep and more rearward on Gen III), and the channel on the top of the BCG was elongated on Gen III. Most notable on the BCG are the redesigned cam pin path, as well as the differences on the method that the recoil system attaches to the BCG.

-Bolt: the Gen I had a 5 lug bolt. The Gen IIIs had a 4 lug.

-Bottom of upper receiver: the Gen III has a longer portion in the front, and there is now a small cutout in this portion. Also a reinforcing plate added on the bottom right at the ejection port.

-Top rail: the Gen I has a traditional pic rail. Gen III has a scalloped rail.

-Recoil spring retaining plate: Gen III has an angled piece added to it which fits into the lower receiver.

-Lower receiver buffer system: Gen I had a red buffer, Gen III has a black buffer shaped differently.

-Lower receiver near rear retaining pin: Gen I had a metallic screw permanently installed into the lower. Gen III has a black rubber piece that is removable.

-Lower receiver above mag well: the angle of the lower was changed on Gen III in this area.

Great post! thanks for the photos. At least the handguards are interchangeable.

My Gen I came with an ambi charging handle. Everything else is the same as yours.

Great comparison! Thank you!

Excellent information! Thanks.