Sig MPX won't fire more than one rd at a time

[Nimitz]

I'm having an issue with my MPX that hopefully someone can point me to the likely cause.  basically the gun will only fire a single round and when i pull the trigger a second time a just get a 'click'.  if I cycle the charging handle, eject the round an new round chambers  I can fire that round but then the cycle repeats itself.  A secondary issue, which may or may not be related is that when I cycle the charging handle the bolt doesn't always fully close and I have to partially reopen the bolt or do a complete recycle and then it fully closes.  I've already taken the gun apart, cleaned it including dissembling the firing pin & gas chamber but that didn't fix the issue. Any suggestions of what to look at?

 

 

[JWS_61]

Have you tried hotter ammo?

[davsco]

sounds like your bolt isn't going back far enough under recoil to reset the trigger.  try a lighter buffer spring or hotter ammo.  i had the same prob with my pcc build and ended up swapping out bolt and buffer to and from another pcc and now both work fine.

[Ming the Merciless]

3 hours ago, Nimitz said:

I'm having an issue with my MPX that hopefully someone can point me to the likely cause.  basically the gun will only fire a single round and when i pull the trigger a second time a just get a 'click'.  if I cycle the charging handle, eject the round an new round chambers  I can fire that round but then the cycle repeats itself.  A secondary issue, which may or may not be related is that when I cycle the charging handle the bolt doesn't always fully close and I have to partially reopen the bolt or do a complete recycle and then it fully closes.  I've already taken the gun apart, cleaned it including dissembling the firing pin & gas chamber but that didn't fix the issue. Any suggestions of what to look at?

 

 

 Sounds like the gas piston (tappet) is sticking for some reason.  Too much carbon in the gas cylinder, or built up on the gas rings.  How hard is it to drive the gas piston all the way out of the gas cylinder?

Edited April 14, 2020 by Ming the Merciless

[TISCHLJ]

Are you using a fast powder in reloads?? The MPX needs enough gas to properly  function. 

[inertia]

Need to know ammo specs, Is this a new problem on an old gun or new gun? Which gen mpx? Any modifications? How far does brass eject if it is ejecting at all? If the gun isn't cycling its either binding from being too dry new and stiff new stiff mags or its not making gas due to ammo or gas system problems, or its not extracting due to ammo or extractor issue. Need a little more info to help.

[Nathanb]

Could also be your cam pin dragging

[davsco]

32 minutes ago, inertia said:

or its not extracting due to ammo or extractor issue. Need a little more info to help.

sounds like it is extracting and loading a new round, the new round just isn't firing.

[inertia]

14 minutes ago, davsco said:

sounds like it is extracting and loading a new round, the new round just isn't firing.

I reread you may be right something off

Lube the hell out of it and test with less than 10 rounds in a 30rd mag and known good factory ammo, bet its dry, new mags or bad ammo.

Edited April 14, 2020 by inertia

[kattman]

Maybe that forward bolt movement is slightly restricted?????on a fresh round going into the chamber.

 

If the extractor has the rubber O-ring around the extractor spring--try removing the O-ring and firing with just the spring.  I had this where the bolt would "almost" go into full battery, enough to allow the hammer to drop, but not forward enough to allow the FP to strike the primer hard enough to discharge.  JMHO--something to try??????  Good Luck!!

 

Kattman from Michigan

[ck1]

If you mean by getting a “click” that the trigger has reset and and then hammer is falling, then it sounds like it’s not going fully into battery again. This could be one or all of these things: you need to run the bolt more wet, need hotter ammo, or your gas piston/rings are gunked up.

The MPX will let the hammer fall but won’t set off a round if the bolt isn’t fully closed, like if you don’t rack it snappy enough at the beginning of a stage, ask me how I know  

[Gregg K]

Do you have the 2 dots lined up on the bolt cam pin?

[pig4bill]

Does the empty eject? If so, it might be the trigger bouncing off the disconnector and following the bolt as it closes. Common with MPX's with modified triggers because the bolt speed is so high. Did you change or modify the trigger?

[jaques]

Go buy a JP or Lead Star and problem solved.

[AHI]

17 hours ago, Nimitz said:

  if I cycle the charging handle, eject the round an new round chambers  I can fire that round but then the cycle repeats itself. 

is the ejected round loaded or empty?

17 hours ago, Nimitz said:

when I cycle the charging handle the bolt doesn't always fully close and I have to partially reopen the bolt or do a complete recycle and then it fully closes. 

this is the root cause of your issue.

[Bdh821]

7 hours ago, Gregg K said:

Do you have the 2 dots lined up on the bolt cam pin?

I’d say check this. Newer MPXs no longer have this issue as the cam pin can go in either direction. 

 

What trigger is in the gun?

 

what generation/model MPX?

[Nathanb]

I’ll add that I have this issue as well with mine now. I just shelved it since I have plenty of other things to shoot.  My next move is to pull the hiperfire 24c and check for damage. Then I’m sending it to rob at in lead we trust if I don’t see any damage. 

[Nimitz]

Thx all ... I’d try to answer  what seem to be the possible big issues:

 

this is a Gen 2 where the gas port has been opened up to Gen 1 standards.  There is a hyper fire 24C trigger.  This is a new problem for this gun.  I’v  been  Running the gun with loads as light as 90 PF without issue before and theses particular rounds are ones from my STI Steelmaster and are about 130 PF since the STI won’t run reliably below that.  Loads are using WST

 

the trigger goes click meaning the hammer is falling so I believe it is resetting

the empty case ejects when the round fires

I probably need to use more lube on the bolt

I recently cleaned the gas port but have had issues before where it was really gunned up ... how often do you clean that gas port.

I’ll have to check the bolt cam pin.  So i guess it’s possible to ressemble it the wrong way and it still goes back together?

5 hours ago, AHI said:

is the ejected round loaded or empty?

this is the root cause of your issue.

 

Loaded

I didn’t’t look to see the position of the bolt when I pulled the trigger for round #2 & nothing happened .  I noticed the bolt not fully closed when i recycled the charging handle to check the round for a possible light strike

 

[inertia]

Fresh recoil springs if your real high round count?

[Bdh821]

18 hours ago, Nimitz said:

how often do you clean that gas port.

 

 

 

I do a complete gas system cleaning about every 1000-1200 rounds. 

 

A clean gun is a reliable gun...

 

 

[Nimitz]

I wish that were the case but I doubt the gun has more than a few thousand rds through it ...

[Bdh821]

A few thousand without cleaning the piston? 

[Nimitz]

1 minute ago, Bdh821 said:

 

I do a complete gas system cleaning about every 1000-1200 rounds. 

 

A clean gun is a reliable gun...

 

 

no argument, so here's the detailed history.   the gun unfortunately doesn't get used that much as I just don't have time to train seriously with more than one gun.  so prior to the FL State Championship in Dec I had cleaned the gun & ops checked it and it worked just fine - about 100 rds.  at the match on the first stage after the first run the gun would not fire at all.  cycle the bolt, new rd in the chamber, pull the trigger & click - nothing.  tried taking it apart but could not see anything broken however after removing the firing pin I could not see through the hole where it would exit the bolt to strike the round and so  we concluded maybe it was gunked up and someone on my squad said you can't get any lube in there - not that i put any in there but I've been told the gun likes to run wet so I've been putting lots of lube on the gun?

 

Finally got around last week to deal with this & took the gun apart ( not the gas port since I had just cleaned it), cleaned everything.  the video I was using from Sig for reference said not to use too much lube (so which is it?) so I didn't.  took it to the range where I experienced the issues I first described.

 

[davsco]

two days ago you said the gun fires when you manually rack it/drop the bolt.  right above you're saying it never fires.

 

are you 100% sure the hammer is falling?  you're hearing it and feeling it?  any dents on the primers?

 

you can def get crap in the firing pin chamber.  when you scrub the bolt face during cleaning, that can force crap thru and into the fp hole.

 

also on your op you said the bolt wasn't fully closing.  if it's a little out of battery then yeah prob the hammer, if it does fall, isn't contacting the firing pin squarely or enough to light the primer, which is a good thing if it is out of battery.  maybe a burr or some crud buildup on the bolt or around the barrel is keeping the bolt from fully closing.

 

look over the trigger assembly.  that's a relatively complicated trigger, maybe something shifted, came loose, whatever, that is slowing it down or impeding it.

[kattman]

I also have had the firing pin and channel so gooed up that I had to pull the firing pin out with a needle nose pliers from using to much lube.  Each attempt to fire produced a trigger release and a hammer drop noise with a subsequent dead trigger.  Clearing the gun by re-charging only produced a same result.  It took a bit to clean/clear the FP hole in the bolt, but I finally could see through the channel hole.  Used carb cleaner and compressed air.  Now I don't lube that area so much.   I am shooting three SBR'ed MPX's and continue to learn their little quirks.  I no longer use any lube in the gas chamber area.  Using ILWT ion bond tappets and change gas rings every so often.  I also do a complete tear down clean before each USPSA match.

 

Just my experience and trying to help.  Good luck.

 

Kattman from Michigan