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Thread: Case head space

  1. #1
    Machine Gunner
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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?

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  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
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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.

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  7. #5
    Command Sgt Major budster's Avatar
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    Re: Case head space

    Excellent explanation, suitcase!

  8. #6
    Rifleman NMC_EXP's Avatar
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    Re: Case head space

    I believe that relative to bolt guns a slightly "looser fit" of cartridge to chamber is preferred for semi-autos in general and service rifles in particular. Cartridge fit includes: (1) cartridge headspace, (2) case diameter (3) cartridge overall length.

    If the cartridge is on the large end it will be hard to chamber. If the rifle is hot and/or dirty it is even harder to chamber. With a floating firing pin such as the M1 and M14 this increases the risk of a slam fire. If you shoot the 80 shot regional course of fire there are four 10 shot rapid fire strings. The gun gets hot and its dirty by the end of the day.

    I use a Mo's micrometer style cartridge headspace gage. I've measured a lot of Lake City Match (both 7.62 and .30 caliber) and a lot of Federal match .308. Unfired it all measures 0.00 on the HS gage. After firing in various M1A match barrels it measures +0.004 to +0.006. These were all commercial barrels and I assume the chamber reamers used were .308 rather than 7.62.

    Some folks use small base resizing dies. I use standard full length dies and set them up to bring the cartridge case headspace back to 0.00 on them gage. That way I can shoot the ammo out of any rifle in that caliber.

    When loading for a gas gun I'd recommend shying away from techniques used by bolt gun shooters (e.g neck sizing and case trim length at max).
    “After all is said and done, successful rifle shooting on the range is nothing more than first finding a rifle and lot of ammunition which will do precisely the same thing shot after shot, and then developing the same skill in the rifleman.” ~ Capt. E. C. Crossman (The Book of the Springfield)

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  10. #7
    Designated Marksman
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    Re: Case head space

    Quote Originally Posted by The Comet View Post
    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?
    Yes, I've played with it. In a service/battle rifle, the cases must be smaller than the chamber to allow for differences in chambers, physical conditions(chamber clean, dirty, half and half), exterior conditions (rain, pouring rain, more rain, snow, ice, duststorms). The idea is rounds downrange at the targets. I've always used small based dies (RCBS X-based) and been very pleased with the results.
    NRA EXPERT(XTC)
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    Shooting a Service Rifle M1A platform.
    I gave the Navy my youth, they gave me my maturity.
    I used to do. Now I teach others to do so the skills are not lost.
    RETIRED

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