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S12 breaking firing pins - need ideas


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Hello all:

 

Have a problem that is stumping me.  This gun has a long history, but seems to have a new problem.  Gun is breaking firing pins after about  50 rounds of # 7 1/2   1200 fps low brass birdshot. 

 

Here are the facts:

1)  Factory twisted wire hammer spring (NOT JTE performance spring)

2) Tromix firing pin WITH Tromix firing pin spring  - these keep breaking.

3) Shortened gas system and short op rod originally done by R and R Targets -- see #4 for the history on why.

4) 18" barrel.   No mid barrel comp.  This was an early R and R gun and originally had a mid barrel comp, but gun was not reliable.  holes were welded up and it's been running fine with no comp.  Has been running fine for several thousand rounds.  This gun does cycle really fast since the gas system is short, but adjustable gas plug is set to minimum to reliably cycle the gun.  This short gas system is the only really "oddballl" thing about this gun -- though keep in mind that this gun, even with the short gas system and no mid-barrel comp,  has been running reliably prior to the current problem.

5)  Stock recoil springs

6) No compensator

7)  Bolt has been completely disassembled and cleaned no crud inside bolt that could make pin stick.  All spotless.

8) Firing pin hole on bolt is normal size (i.e. about 0.098").  Firing pin does not extend farther out of the bolt face  than a stock firing pin.

9) all dimensions of bolt, firing pin, long firing pin section seem to be within 0.004" when compared with brand new factory parts.

10)  As mentioned, this gun cycles wicked fast and throws the shell about 20 feet away -- but it won't run reliably with less gas or heavier springs.  Also as mentioned, it's not using light recoil springs-- these are stock springs (i.e. about 17# 1911 springs). 

 

My theory is that the bolt is cycling so fast that even with the heavy Tromix firing pin spring, the pin is staying extended and is getting sheared off by the next round during feeding.  What my theory doesn't explain is why this seems to be a new problem.  

 

Next steps -- switched an entirely new bolt/firing pin/FP spring into the gun. Taking to range tomorrow.  If it breaks again, it it likely has something to do with the cyclic rate since the entire bolt and  firing pin components are new and the problem stayed with the gun.

I also switched the bolt that keeps breaking the firing pins into a different gun -- If it breaks the firing pin, then the problem is related to the bolt /firing pin since the problem followed the bolt.   Will put a few hundred rounds through each gun until something breaks.

 

Any other ideas suggestions before I go to the range tomorrow?  This one has me puzzled.

 

If all else fails, I can redrill the gas ports and reconfigure the gun back to the original length gas system, but would rather  not go through the work-  especially  since the gun did run OK for quite a while. Just wondering if I[m missing something.

 

 

Thanks all, Malcolm

 

 

 

 

 

 

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The tip of the firing pin is getting sheared?

 

Are there any good dents on the fired shell rims, and in what orientation?

 

Are you sure your hammer is being held by the disconnector?  Your description sounds like your gun is about to start slam or out of battery firing.

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Thanks Capt Nemo --

 

Breaks a where the small pin diameter starts to taper to increase in diameter. 

 

Hammer/disco functions fine.

 

Good idea on looking for the dings -- tomorrow I'll look for dings in the rim and mark the "up" side of the brass so I can see the orientation.

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The only other thing I could think of is strength of the hammer spring.  It might be bottoming the FP on the backside of the FP hole causing the breakage.  What might go with that is the heat treatment of the FP.  Is it hardened to the point of being too brittle?  That would be a Tromix question.

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When you take the bolt apart does it appear to have work done on the bearing surfaces inside of the bolt where the two halves interface?

 

With the bolt apart and looking through the backside of the bolt head, use a light front the front side of the bolt to see if there is any displacement of metal where the tip of the firing pin goes through. The Saiga 12 is designed to be very loose and if there is binding at the bolt face, it could cause breakage of the firing pin. I have seen it with other platforms and can't speak intelligently on whether it is an issue with Saiga 12s or Vepr 12s, but is worth checking.   

 

The only way moving the gas block forward is going to work easily is with a new barrel. If the profile was done correctly after the relocation of the gas block seat, the barrel diameter at the original location will be too small to support the gas block. You will need a new op rod and gas tube as well.

Edited by evlblkwpnz
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Thanks for the ideas -- Took it to the range today.  Tired the New bolt/firing pin assembly in the gun with the short gas system and it broke the firing pin after about 50 rounds.  So the problem is not with the bolt.

 

Took Nemo's suggestion to look for dents in the brass --  No dents on the brass. 

 

Good point Evlblkwpns about the barrel being turned down to allow the gas block to move further back.  So you are right, no way to easily  move it back to stock length.

 

This sucks.

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Strip the gun, and lock the bolt in place without the carrier. Set the hammer on the tail of the bolt. Does the hammer face sit square to the tail of the bolt?

Does the firing pin extend from the tail of the bolt? If so how far?

The hammer MUST strike the firing pin square or it will break it.

The firing pin should not extend past the tail of the bolt. The energy of the firing pin from being struck by the hammer should  be absorbed by the primer,

the firing pin should not bind anywhere in it's channel.

 

You are basically striking a nail with a hammer. Anything that would bend the nail will break the firing pin!

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Can you take pictures of the break?


I think I would be inclined to make the firing pin diameter and that of it's channel bigger before messing with moving the gasblock. If somehow the firing pin is protruding when the bolt comes home, or when the mag is locked in, maybe that could be part of the problem. I.e. a sideload on the pin from the rim of the shell that is being fed.

 

Is there side to side play in the pin, and also can you push on the tail of the bolt and make the new pin extend and retract fully without feeling drag?


Taking what Evl said, if perhaps there was some peening on the bolt face, it could cause a bind at the point the pin passes through.

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I can't help but keep coming back to wondering where the spring is being installed. Are you sliding it on the firing pin before installing the firing pin. That is where it goes. Sorry, I have to ask. I have seen some crazy stuff, so please do not be offended if you are doing that part correctly.

 

+1 on the pics. It is hard to diagnose some things from afar, but don't give up yet.

Edited by evlblkwpnz
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Thanks all: First off, evlblkwpnz , yes the spring is installed correctly-- thanks for checking up on me.  Think it's fixed now.  I checked GOB's suggestions and the hammer did not strike the firing pin squarely.   It was a highly modified hammer that was lightened with a hole through the middle and badly profiled.   I replaced it with another G2 hammer which I profiled to  hit squarely.  Ran 100 rounds through it with no problem.  I case you're interested, took a high speed video of the hammer impacting the FP just to be sure no binding.  Looks good to me.

 

http://youtube/2Pel_ShfDEo

 

Hammer falls at about 1 minute point.  

 

 

Wednesday or Thursday I'll  put another 100 rounds through it -- if no problem then I think it should be GTG.   Thanks for the ideas.  Will let you know how it runs later this week.

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Glad to see that fixed. I always made a point of correcting the hammer angle, but you are the first person I have heard of actually breaking pins with a bad angle.

 

How much was the old one off?

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Glad to see that fixed. I always made a point of correcting the hammer angle, but you are the first person I have heard of actually breaking pins with a bad angle.

 

How much was the old one off?

+1 I'd be thrilled to see what that looked like.

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Thanks all: First off, evlblkwpnz , yes the spring is installed correctly-- thanks for checking up on me.  Think it's fixed now.  I checked GOB's suggestions and the hammer did not strike the firing pin squarely.   It was a highly modified hammer that was lightened with a hole through the middle and badly profiled.   I replaced it with another G2 hammer which I profiled to  hit squarely.  Ran 100 rounds through it with no problem.  I case you're interested, took a high speed video of the hammer impacting the FP just to be sure no binding.  Looks good to me.

 

http://youtube/2Pel_ShfDEo

 

Hammer falls at about 1 minute point.  

 

 

Wednesday or Thursday I'll  put another 100 rounds through it -- if no problem then I think it should be GTG.   Thanks for the ideas.  Will let you know how it runs later this week.

 

Did you know at first that the hammer was modified or only discovered it when investigating the hammer angle?  Assuming it was done by previous owner

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The G2 and also drop in versions made from it already has a bad angle, but I have never heard of that breaking things. I assume that someone made that worse by attempting to reduce friction and grinding an even more crooked angle onto it.

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The entire gun is an abortion. As I said,  It started out life as a very early  R and R Targets race gun with mid-barrel comp and short gas system.  That owner never got the gun to work right and fiddled with it and finally sold it to another guy I shoot 3 gun with.   He welded up the mid barrel comp holes, and it ran OK, but it cycled so fast it was really violent.  When you'd adjust the gas  It would either not run, or run too fast-- the hulls fly about 20 feet away. We've tried every spring combination you can think of as well. The hammer came down at such an acute angle that it almost hit the tail of the bolt, before it hit the firing pin extension.  In addition there was a big hole drilled right in the middle of the hammer to "lighten" it.  The piston rod is only about an inch long and barley stays inside the tube.  I think it's on it's third of fourth firing pin.   As I said, just put a G2 trigger in that gun and it seems to be holding up better-  will know more Wednesday when we'll take it out and put a few hundred more rounds through it. 

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