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Run Away Gun


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#1 Speeddemon02

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Posted 09 December 2015 - 11:54 AM

I had an atypical outing the other day where my MP40 ran away on me until the mag was empty.  After some testing in hopes of narrowing the problem down, this is what I found.  The run away occurs when I use 147gr with a suppressor only.  115gr, 124gr, 147gr without the suppressor and it operates fine.  It also operates fine with 115gr and 124gr with the suppressor.  I checked to see if the bolt was travelling back far enough and it was travelling further back by at least a half inch.  At this point I have no idea what could be wrong, any ideas?


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#2 Paladin601

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Posted 09 December 2015 - 01:11 PM

Maybe the barrel is getting to hot from back pressure and causing the round in the chamber to "cook off" and then run away. Either way sounds as a suppressor problem. Did it ever work properly with the suppressor and those rounds? When was the last time you cleaned the suppressor?


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#3 Speeddemon02

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Posted 10 December 2015 - 10:55 AM

It has worked in the past correctly.  It will do it every time from the first round and on.  The suppressor has about 200 rounds through it.


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#4 hawksnest

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Posted 10 December 2015 - 07:47 PM

I suspect the suppressor is the problem.  I have a Smith and Wesson 76.  Eats anything and runs like a clock until you put a suppressor on it (and a ported barrel) and shoot subsonic ammo.  With suppressor and ported barrel I have failure to eject and/or feed with Winchester 147 grain but it runs well with Fiocchi 158 grain.  I think the ported barrel bleeds off to much gas, so I am going to try subsonic 9mm with a regular barrel and see what happens.


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#5 MG08

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Posted 11 December 2015 - 08:51 PM

Guys have mentioned the ammo - one thing I found in a post sample gun and another guys C&R gun is that when the gun is locked together, and then twisted - say by grasping the magazine well and pistol grip- there is enough play to allow the sear to not catch on the bolt - and the gun runs away.  This may not be the case with your gun, but it is something to watch for. 


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#6 Black River Militaria CII

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Posted 12 December 2015 - 08:29 PM

In regard to the post above about rotating the receiver, a more accurate assessment is that the takedown and sear slots are aobut the same width and on the same axis along the bottom of the receiver tube. If the takedown lug slot has widened from frequent and careless disassembly and reassembly, the receiver can rotate slightly. Rotation of the receiver in the lower can misalign the sear slot with the sear so the sear stops against the bottom of the receiver and cannot protrude through the slot.
However, if this is the case the bolt cannot be cocked for the first round clearly indicating that all is not well.
The malady certainly sounds like short recoil issue to me....... FWIW
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#7 TSMGguy

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Posted 26 March 2016 - 09:01 AM

I'm thinking short recoil as well. The gun was not designed for sub-sonic ammo. The bolt is not picking up the sear, though I've only seen this happen with really light loads. I'd try any good NATO spec M882 ammo before making any modifications. If it functions fine with and without the suppressor, then there's your problem!

 

I can't imagine that a suppressor would make any difference to the operation of a blow-back operated weapon. I'd think that the bolt should already have all of the energy that it needs to cycle before the bullet ever leaves the barrel, suppressor or not.  


Edited by TSMGguy, 26 March 2016 - 09:45 AM.

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#8 ron_brock

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Posted 26 March 2016 - 09:57 AM

I'm thinking its a short stroke as well. The added weight of the suppressor changes the way recoil is absorbed i.e. energy. Also considering the heaviest bullets with suppressor are the issue. I assume these are the lowest velocity rounds you are shooting. That combined with the suppressor can cause the gun not to fully cycle.

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#9 DGinGA

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Posted 03 November 2018 - 03:07 PM

What happens when the 76 is fired in semiauto? If short recoil is happening, the bolt won't stay back.


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#10 huggytree

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Posted 04 November 2018 - 01:00 PM

Mine did a few runaways from rotational twist of the upper to lower

My mag well had a large gap under it as my gun Is a reweld and not welded true. I had my mag well strap cut and a bit taken out to tighten it up

Also had the breech pin slot tightened up

I still have a tiny bit of wiggle but much better and no run away

Lucky the mp40 is easy to control when it dumps

A .308 might have been another story. I have a cetme semi that used to fire runaway FA once in a while. Replaced the trigger group with Hk and its good. First time it happened I went over the berm
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