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Thread: Case head space

  1. #1
    Machine Gunner
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    Apr 2011
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    Case head space

    The case head space topic for some reason attracts my attention, Banbam and I have disussed this several times. He was involved with the Bifg Boy's in the Bench rest crowd.

    As I recall the best results, smallest groups, resulted when the case HS and chamber HS were the same, or near the same.

    That has not worked out well for me in an M1A. I decided to start a record book of the case HS by case Manufactors. To my surprise there was so much difference in case HS between them I decided this must mean it dosen't make a great deal of difference. providing the case HS is not too long, that's the type of difference to avoid..

    This project ended up taking measurements off many rounds of M118 and M852. My system is comparaing, using a RCBS guage.

    The results of different case HS numbers in my rifle show the best accuracy comes from cases that are .003" to .006" smaller then the rifle chamber.

    All of the testing was done using Fed. once fired cases.

    Anybody else playing with this?

  2. Thanks nf1e, Hawk, 4570govt, sergeant major thanked for this post
  3. #2
    Command Sgt Major budster's Avatar
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    Re: Case head space

    The Sierra Manual has, in essence. stated the same facts as you, The Comet. This is why if the same case goes into the same bolt action, it can be neck sized as the case snd chamber are nearly identical. However. In gas guns, SAAMI recomends that cases be Small Based Resized as the camming action is weak based on the gas guns design.

  4. #3
    Machine Gunner
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    Re: Case head space

    Thank you,
    I don't understand the "Camming action is weak" part.. Is this the press handle camming over action? The Comet..

  5. #4
    FNG
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    Re: Case head space

    In an M1 or M14 the force turning the bolt to close the bolt is generated by the operating rod ramp that is riding on the bolt lug or roller. That force generates a torque on the bolt that is then turned into an axial force by the lugs on the bolt riding on their cams in the receiver. Most of this force is used just snapping the extractor over the rim of the case.

    In a bolt action the force turning the bolt to close it is provided by the operator. Not only is this force much greater than that in a auto loader but the bolt handle is much longer so there is a lot more torque available to seat the round. A bolt action can even change the cartridge head space as it is chambered.

  6. #5
    Command Sgt Major budster's Avatar
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    Re: Case head space

    Excellent explanation, suitcase!

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