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Thread: Primer Hardness?

  1. #31
    Join Date
    Feb 2003
    Location
    Wichita Falls, Texas
    Posts
    72

    Lightbulb A little something......

    .....I wrote on the small caliber board, a few years ago.

    A little Jerry Stiller math.....

    A 0.082" firing pin probably has an 0.084" boltface hole. The hole is 0.005412 square inches. At 50,000psi, this results in a total shearing force of 277.1 pounds.

    Now use a 0.060" firing pin, with a 0.062" boltface hole. The hole is 0.003019 square inches. At 50,000psi, this gives a total shearing force of 151.0 pounds.

    So.....for a 0.022" reduction in the boltface hole diameter, there is a 45.5 percent reduction in the shearing force.

    As a reference, and a good read, refer to Precision Shooting magazine, May 2001, page 31, "The Real Reason Primers Pierce" by Jerry Stiller.

    As would be expected, a stronger firing pin spring has little effect.........been there, tried that.

    Why these things only seem to happen with small rifle primers AND small calibers????.....I do not know, or can prove........but I strongly suspect that as the sectional density is lowered, a more "focused" pressure spike through the flash hole is created.....and the firing pin is pushed rearward. BUT.......that's just me.

    BTW.....my old 20BR had about the same situation as above. Quite an eye-opening experience. Greg Tannel reduced the firing pin diameter, and bushed the boltface to a smaller firing pin hole size. The result.........with 40gr bullets blanking at 3400fps.........to NO cratering whatsoever at 4150fps.

    Hope this helps.

    Kevin

  2. #32
    Join Date
    Apr 2017
    Location
    Ames, Iowa
    Posts
    426
    Quote Originally Posted by Kevin Gullette View Post
    .....I wrote on the small caliber board, a few years ago.

    A little Jerry Stiller math.....

    A 0.082" firing pin probably has an 0.084" boltface hole. The hole is 0.005412 square inches. At 50,000psi, this results in a total shearing force of 277.1 pounds.

    Now use a 0.060" firing pin, with a 0.062" boltface hole. The hole is 0.003019 square inches. At 50,000psi, this gives a total shearing force of 151.0 pounds.

    So.....for a 0.022" reduction in the boltface hole diameter, there is a 45.5 percent reduction in the shearing force.

    As a reference, and a good read, refer to Precision Shooting magazine, May 2001, page 31, "The Real Reason Primers Pierce" by Jerry Stiller.

    As would be expected, a stronger firing pin spring has little effect.........been there, tried that.

    Why these things only seem to happen with small rifle primers AND small calibers????.....I do not know, or can prove........but I strongly suspect that as the sectional density is lowered, a more "focused" pressure spike through the flash hole is created.....and the firing pin is pushed rearward. BUT.......that's just me.

    BTW.....my old 20BR had about the same situation as above. Quite an eye-opening experience. Greg Tannel reduced the firing pin diameter, and bushed the boltface to a smaller firing pin hole size. The result.........with 40gr bullets blanking at 3400fps.........to NO cratering whatsoever at 4150fps.

    Hope this helps.

    Kevin
    I thought the advantage of a stronger firing pin spring was in reduced lock time resulting in the FP hitting the primer sooner as opposed to harder which means that the bullet should hit the target a scosch quicker. Wasn't fast lock time one of the extolled virtues of the Rem 788 action?

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