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jhm
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Primer flow
Jul 12th, 2024 at 6:47pm
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I completed my Hepburn build in 32-40 Rem a few  months ago and finally got to shoot it today. Load was 14.5 gr 4198 and a 150gr RCBS plain base bullet. First shot when I attempted to open the breech it was stuck. I am thinking what is going on? With the heel of my hand I bumped the block and after a couple hits (yes it hurt) the block opened. Case extracted easily with no signs of pressure at all. What I did notice is some drag marks on the case which told me the primer was flowing back into the firing pin hole just enough to lock it up. The pin dia. is .093 and the hole dia in the block is .110. I am going to bore it out for a bushing. What would be an adequate pin dia?



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Old-Win
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Re: Primer flow
Reply #1 - Jul 12th, 2024 at 8:44pm
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It happens sometimes with a Hepburn as the block moves straight-up-and-down or perpendicular to the barrel. If you cock it and then trip the trigger again, it will drive the primer back into the case and it should open. It has done that in my original Hepburn quite often. I do think the pin diameter is too small for the hole in the breech5block. I just checked the fit of the firing pin and the hole diameter in a high wall breechblock and the difference was only about three thousandths.
« Last Edit: Jul 12th, 2024 at 9:01pm by Old-Win »  
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GT
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Re: Primer flow
Reply #2 - Jul 12th, 2024 at 10:37pm
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JMH, 
I have good luck with a .078 diameter pin and a hole close to .080 for the Hepburns I've built.  Watch your protrusion and pin shape also, too much at that diameter, not pointed enough and you still have flow - material goes somewhere.  Are you using an original style angled firing pin?
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jhm
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Re: Primer flow
Reply #3 - Jul 13th, 2024 at 7:53pm
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Yes as far as I can tell it appears to be the original. It is indeed tapered at the tip end. I could leave it as is and just bump the pin again but I like you think the pin clearance could be less. Think I will make a new pin a bit larger out of some drill stock and try it. I really hate to bush an original block but I want it to function properly. Try the pin as an experiment but if need be to function I will do the block.



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Bulseyetom
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Re: Primer flow
Reply #4 - Jul 13th, 2024 at 8:22pm
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My 40-70 SS Hepburn does that as well unless I use the military 34 primers.  The solution is to cock the hammer again and pull the trigger.  The hammer seats the primer back to where it started out and the action opens as it is supposed to.  There were several fixes suggested when I asked about what was going on but the simple thing was to just dry fire again if the action did not open.  As I recall the rebounding hammer is the culprit but not being a competent gunsmith I don't remember the exact fix that de Hass recommended.  Tom
  
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JSB30
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Re: Primer flow
Reply #5 - Jul 13th, 2024 at 8:58pm
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A large rifle decapping pin makes a good firing pin. Another way is use the drill bit You drilled the hole with by cutting off the shank.
  
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Dellet
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Re: Primer flow
Reply #6 - Jul 14th, 2024 at 9:57am
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If you have drag marks on the case head, it sounds like the firing pin is hanging up, not retracting.

Stupid question, but are you pulling the hammer back to half cock before dropping the breech?
  
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jhm
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Re: Primer flow
Reply #7 - Jul 14th, 2024 at 10:32am
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The Hepburn has the "rebounding hammer" system so there is no half cock. I think I have it figured out. I turned a larger dia. firing pin but haven't tried it yet. Maybe tomorrow. 




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Re: Primer flow
Reply #8 - Jul 14th, 2024 at 12:11pm
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jhm wrote on Jul 14th, 2024 at 10:32am:
The Hepburn has the "rebounding hammer" system so there is no half cock. I think I have it figured out. I turned a larger dia. firing pin but haven't tried it yet. Maybe tomorrow. 


JMH


Had to look at mine, as I forgot that. 
Still it won’t hurt to double check whether the firing pin is sticking, based on the marks on the brass. 
  
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