SIG MPX Failure to Feed from Mag

[j28s]

Have a MPX with 500 to 1000 rounds through it.  Had issues with FTE so I had the port opened to 60.  Polished barrel throat, dual feed ramp , inside piston, etc. to mirror finish.  Sanded and polished feed ramps on mags.  Happens with different ammo and running 144 PF ammo.  Tried long and short OAL.  It's almost as if the round is catching something and pauses causing the bolt to go up and over the round at a 45 degree angle.  I've noticed if I load the magazines to 25 (instead of 30) it is less likely to occur...but still does occur at least once every two mags.  Load to 30 and it happens some times 2 to 4 times.  Extended mags (40) more.  Anyone else have the problem and correct it?  Sig says simply...send it to them and they'll look at it.  Asking how long it would take and she said don't expect it back for at least 6 weeks.

[Aircooled6racer]

Hello: Looks very dry in there to me. Lube it up very well and try it again. Thanks, Eric

[2011BLDR]

Gen 2 magazines? this is what was happening  when i tried Gen 1 magazines in my carbine. Zero issues with Gen 2 mages

 

Big +1 on lube... when I went to SIG armorer schools in the past we were told Lube should be dripping off any part that moves

 

PF seems a bit low.. WW wight box, Speer TMJ will be 160 PF or so in the MPX (16") I feel like I get a short stroke that results in dot dip at PF below 150, guns cycle and eject brass but I can see the dot dip on bolt close.

 

Keep in mind this is a locked breech system so:

• powder burn rate
• peak presser 
• dwell time

are factors that must be considered and IMO are more important than port size. The fast powders used for blowback system PCC's and Production pistols are not what a MPX needs. 

Edited March 23, 2018 by 2011BLDR

[j28s]

Thanks guys.  They are new Gen 2 mags.  I'll try some more ammo and lube it up.  

[Bigsampson]

Load the mags to capacity and let set for a few days.

Shoot the heck out of it.  Put at least 500 rounds through it of factory rounds.

 Keep it wet.

Make sure the piston is clean and rings are free.

 

Edited March 27, 2018 by Bigsampson

[DoctorH]

@j28s Resurrecting this thread: Did you find the reason and/or the remedy? Mine's acting up in the same way. Similar conditions as described: full mag and the lube having been consumed by the end of a match seem to contribute.

[fbzero]

On 3/23/2018 at 1:15 PM, 2011BLDR said:

 

Big +1 on lube... when I went to SIG armorer schools in the past we were told Lube should be dripping off any part that moves

 

PF seems a bit low.. WW wight box, Speer TMJ will be 160 PF or so in the MPX (16") I feel like I get a short stroke that results in dot dip at PF below 150, guns cycle and eject brass but I can see the dot dip on bolt close.

 

Keep in mind this is a locked breech system so:

• powder burn rate
• peak presser 
• dwell time

are factors that must be considered and IMO are more important than port size. The fast powders used for blowback system PCC's and Production pistols are not what a MPX needs. 

 

Second this, based on my experience this weekend at the Spartan 300.  MPX was a new purchase, ran 200 WWB's through it, followed by 200 of my reloads (134PF in a 5" 2011, haven't chronoed it from MPX yet) with no issues at a practice.  Did a quick clean and lubed it the way I lube all of my other firearms...light film of oil.  Against my better judgement, I opted not to use my proven reliable CK Arms PCC because I wanted to use the MPX - which shoots super soft.  For the first half of the match, I was getting half-strokes that resulted in having to pull the charging handle to free that bullet from the magazine, then strip the mag, shake the loose ones out, and then load and get back to shooting.  Not very fun on stages with 30 second par times and such...

Sig rep was there, so I went and gave him a hard time about it, and he more or less implied that he ran his dripping wet.  Went to a safe area, lubed the hell out of it, and didn't have another malfunction for the rest of the match with what I would consider to be a fairly low PF load for this type of operation...

Edited June 3, 2019 by fbzero

[DoctorH]

The saga continues. With serious lube, she's typically running through a local match with 150 rounds or so. However, today she didn't. After a first failure to feed (of the kind described above) I added some extra oil. After a second failure the next stage, I blasted the action off with Hoppe's Gun Medic and then added oil on top of that, and that seems to have worked. At least I didn't get any more problems, but to get to that I had to go from running wet as hell to running dripping and spewing wet.

 

So something's got to change, though I'm not sure what. Opening up the gas port might be one option.

 

However, on further inspection I noticed something else. This is what the upper receiver looks like inside (still wet from the radical treatment at that match). It seems there's not only wear but a veritable groove in the left-side race surface for the bolt carrier group. The corresponding parts of the bolt carrier group look fine, though I guess steel is harder than aluminum, so they should. Is that normal??? If the bolt carrier is machining off the receiver, that'd explain a greater than usual appetite for lube...

 

 

 

 

[George16]

That’s not normal. The bolt shouldn’t be wearing it off that much especially if it was lubed properly. 

 

Send a picture to Sig and I bet they’ll tell you to send it in for inspection and maybe replacement.

[Neomet]

What George said.  This is not a lube or mag issue.  Get ahold of Sig.  

 

[rowdyb]

And why I sold mine.....

[DoctorH]

1 hour ago, George16 said:

That’s not normal. The bolt shouldn’t be wearing it off that much especially if it was lubed properly. 

 

I wonder what part could even do that. My best guess would be that it's the base of the "op rod" (i.e., what'd be gas key). Other than that there's really no edge in the bolt carrier there that looks like it could do that.

 

Thanks for the advice, guys! I'll keep you updated for the benefit of others trying to play that PCC game.

[George16]

What generation is your MPX? If it’s a gen 1 or 2, the cam bolt has a dimple on it and it has to be oriented in that specific way. If it was installed 180* off, the bolt might be dragging on that side of the upper. It will also feel tight when you try to pull the charging handle. I experienced that when I installed the cam bolt with the little dimple off by 180*. I installed it, double check the manual and it was actually mentioned in the procedure to ensure the cam bolt dimple is aligned/oriented to the mark on the BCG itself.

 

Can you take and post a picture of your BCG showing the cam bolt? You can clearly see the dimple I’m talking about if yours is the gen 1 or 2 bolt.

Edited July 22, 2019 by George16

[DoctorH]

52 minutes ago, George16 said:

Can you take and post a picture of your BCG showing the cam bolt? You can clearly see the dimple I’m talking about if yours is the gen 1 or 2 bolt.

 

It's a Gen 2. I always installed the cam pin dimple facing dimple, as the manual has it. For my education, I just tried it out the wrong way, and the bolt carrier group doesn't even go fully into the upper that way since with the little space at the bottom of the cam pin missing it can't clear the ejector. Obviously, I didn't try violence, but I'm sure it never was in the wrong way since it appears that'd take violence. 

 

When I took the photo, I noticed that there is indeed significant wear on the cam pin (at the side corresponding to correct installation), but I'm not sure how that'd correspond to the wear in the upper receiver. Looking through the back, it'd seem the only way the cam pin can fit through there at all is when it's fully in the upward-facing position, in which case it can't touch the sidewall of the space for the BCG where the groves are. It's all very odd...

 

    

Edited July 22, 2019 by DoctorH

[2011BLDR]

Check to see if the ejector is tight in the upper receiver....one of mine came loose at some point and also gouged the upper..it was the dovetail interface of the spring guide digging in ...I have since changed that upper over to the new competition carrier and spring guide

 

[rowdyb]

Mine had that wear mark, and others.

-I tried factory ammo.

-I tried every combo of bullet, powder and oal with my ammo

-I tried 2 other barrels

-I tried changing mags and mag springs and their various combos

-I tried excess lubrication

-I tried all the polishing of ramps

-I tried all the piston and gas mods

 

Basically I tried every single thing I could think of and my gun under no circumstances could be trusted to shoot 300 rounds without a failure. My failure to feed, eject or extract rate was closer to 1 in 50 rounds through the life of the gun.

 

I do hope you get it fixed and to your satisfaction.

[GregJ]

Rob at In Lead We Trust works wonders with the MPX.  I would send mine to him before I would send it to Sig.

 

FWIW, my Gen 2 runs like a swiss watch, and with my CO reloads. Stock gas port, etc.  Guess I was lucky.  

[George16]

With that type of internal damage to the upper, I will not accept anything other than a new replacement upper and BCG from Sig. I don’t think those gouges can be fixed other than getting a new replacement upper/BCG. Just be aware though that the replacement BCG will be the new/updated style which uses the new black tapered recoil pad so make sure to let them know that you need the black recoil pad. The old gen 2 and Taccom recoil wave spring will work too.

 

I sent mine when the roll pin came out of the gas plug which unthreaded it and cause the system to become undergassed and in turn caused fail to extract. Turn around time was a week including the weekend. Sent it on Tuesday and received the following Tuesday. It came back with the new gas plug, extractor, extractor spring and oring. Works like a charm now with my reloads which I also use on my Shadow 2, TSO and X5.

[Darqusoull13]

2 hours ago, rowdyb said:

Mine had that wear mark, and others.

-I tried factory ammo.

-I tried every combo of bullet, powder and oal with my ammo

-I tried 2 other barrels

-I tried changing mags and mag springs and their various combos

-I tried excess lubrication

-I tried all the polishing of ramps

-I tried all the piston and gas mods

 

Basically I tried every single thing I could think of and my gun under no circumstances could be trusted to shoot 300 rounds without a failure. My failure to feed, eject or extract rate was closer to 1 in 50 rounds through the life of the gun.

 

I do hope you get it fixed and to your satisfaction.

But now you get to spend all that time just loading ammo and practicing with your new PCC 

[DoctorH]

9 hours ago, rowdyb said:

Mine had that wear mark, and others.

 

Did you ever figure out what happened? From looking at the mechanism, it’s a riddle to me how that’s even possible. 

[rowdyb]

nope. Robert at ILWT was an absolute saint with me over the phone but we couldn't resolve it via the scientific method over the phone and me doing stuff on the other end. Not did buying tons of parts fix it. So I sold it with the caveat that I couldn't get it to run right ever.

 

But, I will say this, when it was running it was a super sweet shooting gun.

[Boudreaux78]

I am having some of the same problems with my gun and can’t get them fixed. Sending it back to Sig and see what they say. I won’t run factory American Eagle 124 grain. Driving me crazy. It is a great shooting gun for the two rounds it will shoot in a row. Also having the same wearing issues.

[Canman]

In lead we trust. I had enough with the MPX. Sent it to them and it’s been perfect since.

[Posvar]

Robert is the MPX wizard. 

[Excursion12]

This is depressing. My new MPX competition is doing the same thing. Seems more likely to do it with light powder charges, but I can't run a full mag without it happening. Has anyone had luck with SIG repairing it?