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  #1  
Old 04-11-2013, 09:54 PM
13Fox 13Fox is offline
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Default Cooper M38 firing pin fit

What would the proper fit be for the firing pin to firing pin hole in the boltface be?

My Cooper M38 17AH, which has given me fits since day 1 for piercing and cratering, measures out to:
  • Firing pin = .0667
  • Firing pin hole = .073+ (don't have a pin guage but used a punch measured to this diameter. Hole actually larger than this # but cannot measure without a proper pin gauge. I would add a couple thousandths to this number) Feels very loose when inserting the pin into the hole from the bolt face side...
  • Firing pin protrusion = .047
I am wondering if the fit may be the culprit to the primer problems I have been seeing.


Thanks,
John
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  #2  
Old 04-11-2013, 09:59 PM
TinMan TinMan is offline
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Do you have a set of number drills? If so, try inserting the back of the drill to use it as a pin gauge or to measure the next closest size.
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  #3  
Old 04-11-2013, 10:50 PM
13Fox 13Fox is offline
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No number drills, but I do have a whole box full of "misc" bits I "inherited" from my grandpa! Next closest size is .077 and it did not fit...
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  #4  
Old 04-12-2013, 02:03 AM
SmokinJoe SmokinJoe is offline
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I have had the same problem with my Cooper 38's (3) & had Greg Tannel bush them all. Unfortunately, the Cooper firing pin is case hardened, so when Greg turns it down to .062, it loses it's hardness & hence stiffness, so it wants to bend. Because of this problem cropping up, Greg won't do Coopers, and a number of others, right now. He is planning to get some CNC screw machines that he already has, programmed to make the various firing pins out of heat treated 4340, I think he said. No matter, can't get it done right now. The Coopers seem to have quite a bit of firing pin energy, even with their short pin fall, so I would recommend trying primers with thicker cups if possible.
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Old 04-12-2013, 02:25 PM
13Fox 13Fox is offline
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I have wanted to get the bolt face bushed but did see on Greg's website that he doesn't do Coopers.

I have tried all the primers available in my area, to include: CCI BR4, CCI400, CCI450, WSR, Rem 7 1/2, Rem 6 1/2, Fed 205, Fed GM205M.

I am getting these issues with loads that should in no way crater or pierce, ie 11.0 and 11.5 N120, 1680, and higher too. Heck, I have even gotten good craters with a no bullet-Bullseye fireform load...
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  #6  
Old 04-12-2013, 02:38 PM
SmokinJoe SmokinJoe is offline
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Talk to Greg & see if he'll do a run of Cooper firing pins, or maybe Dave Kiff.
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