Firing Pin Protrusion

Discussion in 'Gunsmithing' started by Edd, Jul 1, 2018.

  1. Edd

    Edd Well-Known Member

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    Is .040" enough protrusion for a bolt action rifle with a 1/16" diameter firing pin?
     
  2. J E Custom

    J E Custom Well-Known Member

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    Even with little or no head space, it is marginal in my opinion. If you are using standard rifle primers you may not have trouble, but with magnum primers with thicker cups the odds go up.

    I consider .045 minimum and .055 maximum. rifles with lots of head space (Normally old military rifles with thousands of rounds through them can stand more) because the case moves forward when struck and this reduces the impact value of the firing pin.

    Try it and see if it ever misfires before hunting with it. It only needs to misfire once in my opinion to need some attention. A good smith can machine a little of the firing pin stop to increase the protrusion.

    J E CUSTOM
     
    Last edited: Jul 2, 2018
    LoneTraveler likes this.
  3. MagnumManiac

    MagnumManiac Well-Known Member

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    I agree, .040” is too short.
    I actually like .050”-.060” for my own builds, for that added security.
    So my opinion is different to J. E.

    Mausers often have .065” as they come, which isn’t a problem in my experience.

    Cheers.
    :)
     
  4. livetohunt

    livetohunt Well-Known Member LRH Team Member

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    I measured my bergara recently, it’s a short action with a 2mm metric firing pin, but it’s .047” protrusion
     
  5. J E Custom

    J E Custom Well-Known Member

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    The pin protrusion is only one part of the equation. And this is why.

    There so many factors that effect the firing pin performance with a firing pin that has the proper/recommended protrusion that it can pierce the primer or not even set it off .

    The heavier the firing pin, the slower it is and requires a heavy spring
    The impact travel may not be as much as the protrusion dimension.
    the same is reversed with shorter lock times and lighter firing pins. They may not need as much protrusion to function properly.

    One such instance was a super light firing pin that had .057 thousandths protrusion that had a history of misfires. Wanting to know why, I modified an Old tool for measuring protrusion by machining a point on it. After zeroing it on the bolt face, I then moved to the pin strike indentation. The "Actual" depth of the strike was less than .040 thousandths. we then changed back to a standard weight firing pin and the strike depth was back to almost the same as the measured protrusion. (.054)

    This phenomenon is not unusual for heavy firing pins ether, the heavy firing pin is slowed by the primer and may not penetrate as deep as it is set. so If you are having problems, measure the actual firing pin impact and this should tell you where the problem is. You may need to change firing pin spring, the firing pin it's self or make sure that the firing pin assembly if free to move it the bolt. (This is a common occurrence in ARs that foul the firing pin, and bolt actions where the spring snakes down the firing pin and rubs on the inside of the bolt.(Slowing the firing pin reducing the strike energy).

    The recommended protrusion is a good start and for the most part sufficient, but sometimes, some trouble shooting is required. Measuring the actual strike depth will tell you if you have a problem or not. Sometimes this measuring process is necessary if you change primers because of cup thickness and hardness.

    I am not sure if there is a desired depth for the pin strike but it does have an effect on the consistency of the ignition of the primer depending on the construction of the primer and the compound used.
    (If anyone knows what this is please educate me).

    J E CUSTOM
     
  6. Edd

    Edd Well-Known Member

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    http://www.dtic.mil/dtic/tr/fulltext/u2/a114616.pdf

    2.4.3 Primer Cup Indentation - Any discussion of firing pin design is not

    complete without mentioning primer cup indentation. Uncontrolled indentation

    can lead to penetration of the primer cup, which results is some back-venting

    of the primer as it functions. On the other hand, too small an indentation

    may not adequately collapse the cup against the priming composition and the

    anvil, resulting in a no-fire. It has been found that a depth of .020 to .025

    inches should be considered ideal for small diameter (pistol type) primers,

    while a figure of .025 to .030 inches should be used for large rifle type

    primers. Although lower indentations may be acceptable, a minimum of .012 inches should be observed with conventional firing mechanisms, since anything

    less is a "light blow" and may be the cause of a malfunction.
     
  7. J E Custom

    J E Custom Well-Known Member

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    Thanks ED
    That is exactly what i needed and has cleared up some things for me.
    It is a lot of good reading so maybe we can ask Len to make it a sticky in the reloading section.

    Thanks again:)

    J E CUSTOM