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Thread: Firing pin protrusion

  1. #1

    Firing pin protrusion

    Whilst becoming an internet expert on rifle rebuilding (sic) I discovered a Midway video where Potter measures the amount of pin that protrudes on firing.

    having just rebuilt a rifle and test fired it on a empty cartridge with a fired primer I was concerned about the depth of the strike in the primer.

    I have just compared the .270 parker hale bolt to my .243 parker hale bolt and they do have quite different pin protrusions.

    Is this likely to be a problem?

    .270 - 1.48mm and 1.7mm (manually pushed out)
    .243 - 0.9mm and 1.16mm (manually pushed out)

    1.7mm seems an awful lot to protrude


  2. #2
    Not being an expert (and I'm sure there are some on here) you said you test fired it? I think the only issue would be if it pierced the primer - other than that I doubt there would be that much of a problem...

  3. #3
    I test fired it into an already fired primer in an empty.
    It made a considerably deeper dent than the rifle that had fired the round.

    I appreciate there is no charge to work against the pin but it just looks a lot


  4. #4
    I'm surprised no one has responded to this.... Oh well. I tried this on an already fired cartridge. Same rifle and you can see the one struck again was much deeper and had bent the primer in. I dont think this is a good test to see if your pin protrudes too much. I think if it isn't piercing the primer its not much to worry about.
    Click image for larger version. 

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  5. #5
    great, thanks for that,

    was wondering if I was going mad!

  6. #6
    Think about it, the fired case will have expanded fully to the chamber, this may then result in it not sitting as far into the chamber as a resized round. this in turn will lead to the deeper strike you have noticed.
    I'm sure there will be a technical manual available some where that gives the min and max tolerances for any part of the rifle and action, ask someone like Fultons at Bisley.

  7. #7
    it is not so much the dent size that concerned me but the comparison between two identical actions.

    an extra 70% of pin protrusion is an awful lot when the 0.9-1.1mm on the compared action already leaves a normal strike on the .243

  8. #8
    Quote Originally Posted by murrayb View Post
    I'm surprised no one has responded to this.... Oh well. I tried this on an already fired cartridge. Same rifle and you can see the one struck again was much deeper and had bent the primer in. I dont think this is a good test to see if your pin protrudes too much. I think if it isn't piercing the primer its not much to worry about.
    When you fire a rifle, it takes just a few milliseconds from when the firing pin is released to the bullet exiting the barrel. A lot happens in those 4-5ms.

    The sequence is this: the firing pin strikes the primer thus crushing the primer compound against the anvil; the primer compound, which is a percussion- and friction-sensitive high explosive, explodes; the explosion generates gas at high pressure within the primer cap; the high pressure forces the primer cap to back out of the primer pocket, the primer cap is stopped by bolt face; the firing pin spring holds the firing pin protruding out through the firing pin tunnel; the main charge of propellant ignites and creates high pressure within the cartridge body; the friction between the bullet and the cartridge case neck is overcome and the bullet moves forwards; in response, the cartridge case moves rearward and, in doing so, re-seats the primer cap; the cartridge case presses against the bolt face; the primer cap deforms against the bolt face and firing pin. This is why the indentation appears to be small - the primer cap is moulded against the firing pin and bolt face.

    The apparently larger firing pin impression on a fired case is the result of there being no high pressure involved. A misfire also has an apparently large dent. An analogy might be this. Throw you body against a van's rear door, the indentaiton will be large. Now take a hammer to the door. The indentation will over a small area.

    --JMS

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