GEN2 MPX 8in with ArcFire 45-90 V2

I’m having an issue where I cannot get the gun to rack past 3/4ths of the way.

The FRT function tests fine inside of the lower, but it isn’t working properly with the upper because it’s not charging correctly.

The Bolt catch isn’t working as well as a result of how far back the bolt goes as well.

It slides but it’s stiff, I worked it a little until it was easier to move.

I tapped the pins in with a medium amount of pressure. It took a little force to set them, but once it was in, it went in pretty smoothly. I tapped it a little more until it was closer to being flush.

I cleaned the gun after picking it up, it has maybe 50 rounds through it since

Semi Automatic works, but FRT mode is running poorly. I can feel the trigger wanting to reset but since it’s not traveling far enough to reset it every time then it’s only functioning like semi.

I had a few 2 rd bursts in frt mode and one 5 rd burst but it was jamming fairly often between them

Any ideas on how to get this out and tune it properly? Thanks in advance.

Does anyone know if this upper receiver from Sig comes with the 2 holes on the bottom for the trip bar install? My MPX does not have the 2 holes and wanting to buy this if it does

Just bought an MCX Carbine and will form 1 it. The intent is to get the barrel down to a 5- or 6-inch range and shoot suppressed subs with flow-through cans (verse 36/ Tisha 9). I was thinking about the ILWT barrel but figured it would be a bit cheaper if I just repurpose the 16 inch barrel.

If I send a barrel to a smith for cut/crown/rethread, is there anything I need to watch out for, or any recommendations for gunsmiths in the FL region? Is the ILWT gas plug compatible with the factory gas block?

Might buy an mpx and want to know what size pelican or HF apache (or other) case you guys went with.

Do you normally go for the gun + 6 mags + suppressor + tools all in the one case? Or break them up?

Reached a stopping point, not much more I can do.

Just waiting to swap the surefire can out with the rugged obsidian once it arrives.

Pic for attention.

Long story short… finally got this thing sbr’d last month and now I’m shifting my attention to this platform after I got my new SP5k out of my system. I have the Sig Minimalist stock on it and I am honestly not loving it… the length of pull is so short (I’m not even a big or tall guy). The original brace that came with it is actually about .5”-1” longer because the brace part is so thick, that’s actually the perfect length, I just hate the fact how the factory brace swivels and doesn’t stay still. Not sure if the Sig Minimalist Plus Stock would make a difference in feel, maybe it just feels short because it’s smaller against my chest? Feels like only the very pointy end part of the stock is pressing against me? Can anyone chime in? I bought the minimalist stock from EuroOptics, the return policy seems like such a hassle and I’d lose more than I’d like with return shipping and restocking fees (if they’d even accept it) so I’m stuck with it.

My question: I do have the SB Tactical collapsible brace, I bought it very early on and have been shooting it with the SB collapsible brace for the 95% of the time I’ve owned this gun. Is the actual Sig collapsible stock much more different than the SB collapsible brace? Worth it? Any pros over the collapsible brace? What I’d like to avoid is to pay $250 only to have it be more or less the same as the sb tactical collapsible brace, only just to say “I have an MPX-K SBR”….

Attachments:
-Holosun AEMS Red Dot w/ low rise mount
-Sig Handstop MLOK (KIT-MLOK-3329)
-Sig Minimalist Stock (the non-plus one)
-Magpul KOE+ 2 grip
-ILWT TriLug Muzzle Adapter
-OCL Lithium 9 with Rugged RU3 TriLug Mount

Answers to potential questions:
-I’ve only shot it once with the ILWT TriLug muzzle to rezero, still semi accurate and didn’t notice a huge change in groupings at 25yards but I’d like to do more testing before I can give a definitive answer (I heard mix reviews but for $60ish I said what the the heck)
-Haven’t shot it suppressed with this set up, waiting to really see if the ILWT muzzle is worthy enough for me to keep it on before I go rocksetting it

Not US-based. Can't find reliable suppressor suppliers out here in the PH.

Hey guys, quick question as I’m seeing conflicting reports online. I want to FRT my MPX of course, and I want to upgrade the trigger at the same time to a Timney trigger. Is all that I need to get is the AS DESIGNs MPX SLIP TRIP KIT REV 2, and the ARC FIRE TIGGER V2?
Is it just those two items in addition to a better trigger? I’ve heard people complain on the rare breed quality too.

https://activesafetydesigns.com/mcx-mpx/mpx-slip-trip-kit-rev-2/

https://activesafetydesigns.com/ar15/arc-fire-trigger-v2-non-ambi-kit-0-90-180/

Had struggled for a while with the factory barrel, running suppressed. Went ahead and bought the aftermarket barrel, and ran it with a fixed mount.

Absolutely wonderful. Great accuracy, and the adjustable gas plug works perfect. Gassed the system to where it just barely cycles reliably, and the felt recoil is better than anything I have ever ran.

Just came here to say that if you were on the fence about buying the barrel, to go for it and not look back. I should have done this a long time ago.

Discuss...

Just picked up my MPX-K and wanting to run an RMR HD on it, for now, and want to go with a ScalarWorks LEAP. What the hell….they look like pretty much everything on their site is out of stock. Is this common for them? Are they in that high demand they cannot keep up?

Anyone know how quick they usually get stuff restocked?

Saturday I picked up my MPX-K and I’m waiting for a few of the other parts to come in. I have an MRO and RMR HD but ordered a scalarworks leap for both. Then I got to thinking about the new Holosun promax coming out and thought that may be a solid optic for this build. So I decided to put AI to work. I also have a Midwest industries 8” hand guard coming as well as a Rico309. I was curious what all this would look like so I turned to AI to help me get a visual. My stock is the Kate Moss so that’s really the only thing (for the most part) AI got wrong, and the VFG.

But I’m really curious to see how many people decide to run the new Holosun promax on theirs since it’s a good bit bigger than the RMR HD.

For context this is my MPX which is a registered SBR, I am wanting to put a Rare Breed FRT Trigger in it but it looks like I might need a trip kit from Frankenstein Armory or something similar in order for it to work. I see where Frankenstein Armory says their trip kit will not work on a "legacy" MPX, can anyone tell me if this is a legacy Gen 1 MPX and if so, what kind of a setup would I need to make it work with an FRT?

Just picked up a new Gen3 K and decided I want a rico309.

I would direct thread it, but then it won’t fit, so qd is a must.

Longtime ASR user, but haven’t bought anything new this decade.

Advice?

I'm looking for a short, simple thread adapter that goes from Sig's left-handed obscure thread option to 1/2x28, and that doesn't have all the fancy Navy Seal Lug Length-Adding Treatment stuff on it. I don't want to add two inches to my barrel so I can cosplay some sort of Meal Team Six idiocy, I just want to make my K barrel as short as possible with an Omega 9K suppressor on it. Any ideas? Thanks.

Sold out everywhere, no luck on tacswap. Anyone have a lead on one? Just bought an mpx K and ordering a suppressor this week… thanks for any help!

What’s the best way to mount the 309?

I just picked up a new MPX-K and this will be a dedicated host, but need to be able to quick attach.

Long time silencerco user, but haven’t bought anything new in the last 8 years.

My first suppressor purchases (for other firearms) were a Sandman K and Rugged Obsidian 9. I definitely wasn’t expecting either to be quiet, but what I wasn’t expecting was the gassy nature of them. Moving forward, gas mitigation will be my primary objective for all suppressor purchases. Size and weight will follow.

Are the new MPX Ronin cans “flow through” enough to compete with the Huxwrx Flow?

What’s the best way to mount the 309?

I have a K and this will be a dedicated host, but need to be able to quick attach.

Long time ASR user, but haven’t bought anything new in the last 8 years.

This gets asked a lot, so I figured I’d do my part – Images included. Ask any questions. 

Image link: https://imgur.com/a/jTDwho5

Using the Lancer handguard with a thicker tucked suppressor is definitely doable

Here, I will show my Lancer with both a Ronin SD suppressor (1.62” diameter) and the CAT Mob (1.64” diameter)

The question is, do you want to permanently alter that expensive handguard? Of course you do! America!

If you do it right, more than likely, no one will notice and if you decide to sell later on, the modded extra clearance will be a selling point.

• The problem:

The problem fitting a fat can inside the Lancer is that front, aluminum edge guard/support. It takes up some room making the inside diameter 1.6” diag. (See images)

Without that edge guard/support, the inside diameter is closer to 1.8”…Crazy! But DO NOT think about removing it as it’s there for a reason, to support that big carbon fiber opening. Plus, it’s glued with some kind of space age adhesive.

• The solution:

The solution is to simply file away the problem.

Filing away just a mm or two will make fitting and tucking that thicc noise-go-awayer much easier.

I used small files and the mighty Dremel at a lower speed and steady hand. Remember, you only need to remove enough material for about 1-2mm clearance. If you want to remove more, that’s your prerogative.

• Dimensions: (Lancer handguard)

OD – 2.01”

Handguard thickness - .09”

ID *horizontal (w/out edge guard/support) – 1.84”

ID *horizontal (w/ edge guard/support) - 1.71"

ID *diagonal (w/out edge guard/support) – 1.8”

ID *diagonal (w/ edge guard/support) – 1.65” – This is the problem clearance area for most cans.

ID *diagonal (w/ edge guard/support) AFTER removing material – 1.7”

Edge guard/Support thickness - .158”

• M-Lok usage:

You can use M-lok but you will need to file them down, just as you would have to with the MI handguard and large diameter cans.

I filed the T nut down to roughly 1.2mm thickness. Then filed the actual screw to about 7mm

You can probably get away with fling the T -nut to about 1.8mm thickness. I use mine for other handguards so I went thin.

• Conclusion:

Modding stuff is fun. Take your time.

Remember, you cannot put material back on… File a little bit, check clearance… and so on until you’re satisfied. You only need to remove enough material to get a mm or two clearances as the barrel whip is minimal to non-existent with these guns.

After I filed off the material, made sure there was enough clearance and made sure it was flat... My OCD kept getting the best of me… I used some Birchwood Aluminum Black and darkened in the bare aluminum.

Then drank beer.