I went shopping for a Banshee yesterday and came across a used MPX. I spent a few minutes trying to do a little bit of research on my phone, but it was still pretty much an impulse buy. From what I can gather it is not a first generation gun. It has the ambidextrous latch and the key mod handguard. It also has the 8 inch barrel. It appears to have fixed factory Sig flip up sites. What is your recommendations for a brace? Would you go with a factory Sig folding brace, or with the SB tactical version? I have a Trijicon RMR that’s not in use so I’m thinking I will use that for an optic. Any suggestions on anything related would be much appreciated!

So I like the SB tactical collapsing brace. The Kate Moss slim folding brace is also a nice stock for that. I changed out the trigger for an MPX specific CMC Light flat trigger which is fantastic. I like the 8”, keymod less so. Get good magazines with metal feed lips - lots of literature on the right ones. Beyond, that go Wild and have fun. Have the Trijicon Red Dot on mine with Backup Irons, and a light as a I have in the bedside safe. Then train with it, which I don’t do enough of…

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Picked up a used MPX, now what?

Try and find ammo for it?

I've got a Gen II MPX, keymod and 8" barrel, as do 2 other guys I shoot with every week.

We all use the factory brace and have added the Caldwell Brass catchers for range use as we all reload.

Great guns and accurate, too.

Congrats, the MPX is too much fun. The MRO will work great on it. I am using the telescoping brace, it is not the most comfortable but for concealment and transporting its perfect. OEM trigger is fine until you compare it to the aftermarket crowd. Timney MPX trigger would be a good route to go.
I changed my charging handle out to the Radian Raptor because the OEM felt unstable and weak.
The MPX makes a great suppressor host if you want to go down that rabbit hole.

Congrats. That configuration is the sweet spot for the carbine, IMO, and it's a shame they don't offer it anymore. Keymod is a drag, but fixable. My unit has been 100% reliable with all ammo, even handloads, and is damned accurate with those handloads.

I can't suggest the brace, since mine was bought was a straight pistol with no brace, and now it's on a Form 1.

I also changed out the charging handle. The brand escapes me, but it's one that lifts it up to prevent galling the receiver (never had the problem myself, but the factory charging handle was just so-so).

Also added a Kaw Valley linear comp just a few weeks ago. Does seem to quiet the report down a bit and makes it a bit friendlier when shooting on a line.

Upgrades I plan but haven't gotten to yet:

A trigger replacement is definitely a good idea, IMO, because the trigger is crap. Maybe my sample size of one is a bad one, but I imagine there's a reason they now come with Timney units from the factory.

Finding a Mid West M-lok handguard that's longer than the factory (like 10" unit) because I eventually will be running a dedicated suppressor on the gun and want more space for my hands.

IMO slap the MRO on it and have fun. I use the SB tactical brace but with the coming sh** storm not sure what to advise on that. I've never had an issues getting anything I needed in keymod, though pretty much like everyone else I prefer mloc. But I wouldn't (and won't) pay to change that till it was a barrier. I use it in gun games and use keymod to picatinny to get my light on it. I use the bobro MRO mount since you cqn pick a side for the attachment crap.
The trigger is usable but the worst part of the setup. I've have a bunch of these and have put in aftermarket Geissle, Timney and Wilson. I prefer the wilson overall. And I have broken one of the two Timney's I have FWIW. Mags are pricy but that's just the price of admission, they run well.
Have fun.
Its very nice.

I'm "that guy" who wound up modding my Gen2 MPX quite a bit. Honestly it really didn't need most of it, but most of the changes were things that I thought would make the gun work better for me. So below are the things I've changed on my 8" Gen2 (sort of arranged in order of importance, but also not really by the time I got to the bottom):

- Aftermarket charging handle (Raptor). Better clearance to address the issue of receiver wear caused by the OE charging handle. Frankly the properly installed aftermarket CH still occasionally scratches against the upper receiver, but the actual cutting damage is more cosmetic to the finish than what the damn factory handle was doing to the aluminum itself. This change out is something that I would consider a must-do.

- Timney 2-stage trigger, rated for use with the MPX. I prefer 2-stage triggers in my ARs and the factory MPX trigger was always kind of sucky. So once the Timney became available I made the transition. I honestly wanted the announced Geissele MPX unit, but that had not gone above vaporware status for so long that I caved and pounced on the Timney. Of course it was only six or so months later that Geissele had HIS MPX trigger to market. Oh well. Grr. At least the Timney is a darn good trigger. Not quite SSA-E level, but still pretty darn good, especially for 9mm subgun.

- Replaced the ambi safety lever. Went with a 45° action, right-hand (left side) only configuration setup. Used a Strike Industry part that actually could be set up to be ambi, but comes with the parts that allow for right-side control deletion. I hated how intrusive the factory ambi lever was so this change was a no-brainer to me. Did require a fair bit of fitting to make it clear the shape of the MPX's upper receiver.

- Switched over to M-LOK from Keymod. I finally surrendered to Magpul M-LOK gods. Bought a Midwest Industries handguard and made the change last year. Of course all of my Keymod add-ons (QD sling attachment, hand stop, M1913 rail for light attachment) were instantly made obsolete in doing so. Still trying to decide if I want to continue to use my Streamlight TLR-1 800 lumen or switch to a torch-style light with pressure switch. Someday I will make up my mind, or so I tell myself.

- Changed the factory folding "SIG" pistol brace. My MPX came with that big fat original brace (the one that ATF actually approved), which I swear had to have been made by SB Tactical because of the close similarities of it to subsequent SBT braces. The SIG brace folded nice but the brace's arm clamp is one bulky part and not the most pretty-looking by a long shot. Then again first-gen designs seldom are. Went to the SBT PSB telescoping brace, chosen mostly for its aesthetic look than anything actually practical. Downside of the PSB is that in its collapsed position it makes the MPX longer for transporting, which required me to find and buy a new transport backpack. The things one does for "cool".

- Nuked the factory iron sights for something less "bulky"-looking. SIG supplied my MPX with those cube-like folding iron sights that also came on my early build Gen2 516 AR. This change was purely about aesthetics, because the sights did otherwise work reasonably well. They just looked dorky. Went with the Lowrider from Bobro; super unobtrusive when folded down flat but not the best made sight around. The metalwork seems spot-on but the pivoting parts has a lot of slop that Bobro CS never did have a solution for. On a 100yd and in gun it's really not that big of a deal, but these sights were originally intended for use on an AR-15, something whose abilities that CAN be stretch out well beyond 100 yards. I wouldn't recommend these unless a uber flat backup sight is a driving desire for you. Because 'dorky' affects more than just aesthetics, as I found out.

- Changed out the birdcage flash hider for a HK-style 3-lug muzzle device. Done for my suppressor. A no-brainer for me, YMMV.

- Swapped pistol grips. I had the urge for a G10 solution, but this was before the time Hogue started servicing the AR market. A now-defunct company out of Oregon was making a design that utilized an aluminum subframe sandwiched between G10 grip panels, similar to what could found on a CZ75 or 1911 handgun. In use I've come to really like this design and now wish I had the foresight to have bought more of these grips...or had a CNC machine and the skills to make more of them myself.

- Went with a red dot optic. First kept it in the family with a Romeo4. Moved quickly over to an Aimpoint T-1 over a Larue mount. The Romeo was sort of okay but definitely kind of chintzy (never could get its solar panel to function right), so I went for a durable Swede instead. However I'm now mulling over going full circle and switching to a Holosun with the circle-dot reticle. I currently use that reticle with my SP5K PDW and it's so much sweeter than a simple dot with my astigmatic eyes. I've gotten to the point where I'd gladly sacrifice toughness for the ease of understanding what the hell my eyes are seeing.

The only other things I've added is a Blue Force Gear Vickers sling and sucked it up and bought a dozen Gen2 30rd mags.

I really appreciate all the information! I am for sure going to get a folding brace, and I’m pretty sure there’s going to be the factory style SIG folder. That is going to be first on my list. I have a Trijicon RMR that is not in use, so I am going to get a mount for it and give it a try. If I don’t like it I will go with the MRO. I will also add one of the Radion raptors. I have a couple of those on my AR’s and I really like them. I will probably leave the Sig BUIS for now.

Have any of you guys added color to your MPX? Mine is all black, and I know some of the copperheads and MPX were FDE? I thought about going with an FDE brace and grip, and rail covers, maybe a FDE light. Or possibly gray. I’ve looked for some pictures online and didn’t really have any luck. Thoughts?

Form-1 To Sbr and lots of ammo to shoot

Keep the gas system components reasonably clean!

I can say from personal experience that I didn't like an RMR on it. The MRO seems mint. All FWIW.

I still left the original Sig folding brace on my Gen2. I added a binary trigger, a Griffin Armament tri-lug adapter for a Rugged Obsidian suppressor. I also pulled off the handguard and added a longer one to bury the suppressor under the handguard. Added a Sig Romeo3 red dot optic, which is basically the Romeo 1 on a one inch riser. Bought 4 extra mags, a requirement with the binary trigger.

Brace etc is all personal preference.

2 suggestions:

1) You will eventually need to remove the barrel to clean and maintain the gas piston system. Re-installing the barrel requires proper torque values. In Lead We Trust (ILWT) sells a pre tensioned torque key for $25 that is well worth the $$$

2) If you are going to shoot suppressed consider an ILWT replacement barrel. They have a built in 3 lug and ate threaded 1/2 -28 with a square shoulder instead of the proprietary tapered shoulder on the SIG barrels. it will make your life a lot easier.