acceptable headspace growth for a 7stw???

Discussion in 'Gunsmithing' started by oidg, Jul 18, 2014.

  1. oidg

    oidg Member

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    I recently had a reputable gunsmith (that I'd rather not call out at this time) rebarrel a 7stw for me. I had shot out the previous barrel. The rebarreled rifle shot decent, about .75 MOA at the range today, but I expect it to shoot much better, as it was a sub 1/2 moa gun prior built on a stiller action. After getting home from my first trip to the range with it, I got out the Hornady headspace gauge. Growth from new unfired brass to once fired measured .020. I'd like some feedback from you guys before I bring this up with the smith. What is acceptable headspace growth? thanks
     
  2. J E Custom

    J E Custom Well-Known Member

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    It is a different chamber and reamer that is the difference.

    Head space is usually from .0005 to .003.

    Two belted chambers with the same head space can produce two different length cases because they head space on the belt.

    Once you fire form they should not change any more unless you have the dies set incorrectly
    or you are over loading the case/cartridge.

    The SAMME dimensions for a 7mm STW case length are 2.830 to 2.850.

    So once you fire form the brass in you new chamber it should not change (You will have to reset your dies).

    So if the chamber is .020 longer it is still within SAMME specs.

    J E CUSTOM
     

  3. MagnumManiac

    MagnumManiac Well-Known Member

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    What J E Custom said is correct, but you must realise there is no SAAMI head to shoulder datum measurement for headspace due to the belt being headspace control. This means that the shoulder position of the brass does not have a minimum dimension held to tight tolerance like the headspace dimension is, .020" is not a lot, I've actually seen as much as .065" difference.
    As long as you don't bump the shoulder back any more than .002", you should have no problems with your brass. You can always fireform your brass if your not happy with it.

    Cheers.
    gun)
     
  4. oidg

    oidg Member

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    Thanks gentlemen. I love the wealth of information offered by this community.
     
  5. WapitiBob

    WapitiBob Well-Known Member

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    Just shot my STW last night so I went out and checked. New brass grows in OAL from 2.838 +/- to 2.848.

    Shoulder to case head grows from 2.248 +/- to 2.262 (comparator insert numbers). I use a 7mm Mag fl sizer as a shoulder bump die, and I have the die set for a .002 bump but it doesn't set it back till about the 3rd firing.
     
  6. J E Custom

    J E Custom Well-Known Member

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    Just a little history on the belted cases.

    Originally the belt was developed by Holland & Holland for dangerous game hunting where being able to reload quickly and trouble free could be the difference between life of death.

    The belt holds the case head at a safe head space while the case body can be much smaller than the rest of the chamber. This assures that a round can be chambered even in a dirty or fouled chamber.

    If you over size and bump the shoulder on a shouldered case, you can have excessive head space and a dangerous situation.

    The belted case was not designed with reloading and brass conservation in mind, Just safety.
    But if loaded correctly the belted cases are know harder on the brass than the non belted cases.

    Most belted cases come sized much smaller than the chamber for this reason. when fired it is essentially fire forming to the chamber and becomes a shouldered case unless you size it back to its original dimensions.

    Most successful shooters of belted cases do minimum sizing or neck size only (This saves brass and
    normally improves accuracy).

    J E CUSTOM
     
  7. Edd

    Edd Well-Known Member

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    There is a base to shoulder datum spec for a 7mm STW chamber and other belted case chambers. For the STW the minimum is 2.4697" and the maximum is 2.4797". That is measured at a shoulder diameter of .420".
     
  8. MagnumManiac

    MagnumManiac Well-Known Member

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    Read my post again, I made no mention of the CHAMBER datum, only the dimensions of the BRASS, which are NOT held to these or any other dimension, if they were, they would all measure the same from every manufacturer, which they DON'T.

    Cheers.
    gun)
     
  9. Edd

    Edd Well-Known Member

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    There is a base to shoulder datum spec for 7mm STW cartridge and other belted cases. For the STW the minimum is 2.4538" and the maximum is 2.4608". That is measured at a shoulder diameter of .420".

    So based on the SAAMI specs, the acceptable growth from unfired new cases would be between maximum of .0259" and a minimum of .0089". The .020" the OP had is within that range.