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Help on shoulder bump for 7MM Rem Mag

 
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  #1  
Old 09-04-2008, 07:40 AM
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Help on shoulder bump for 7MM Rem Mag

For a 7mm RM, Win brass, I measured about 20% sample after 2 firings. Measuring from the head to the datum line with the Hornady case gauge, they ran from 2.118" up to 2.121", with an equal number at each length in between. How far should I be bumping the shoulder back?
Thanks for your help, and this is a great forum, I really enjoy it and get a lot of good info.
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Old 09-04-2008, 08:04 AM
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Re: Help on shoulder bump for 7MM Rem Mag

You want to let your brass grow unitl it has a slight crush fit before bumping the shoulder back. The actual measurement that you will push it back to is individual to your rifle chamber so someone else's measurement may or may not be applicable.

To check for crush fit just chamber a fired unsized case and compare the amount of force it takes to close the bolt as opposed to the amount of force to close the bolt on an empty chamber. To chamber a case in a controlled round feed rifle you remove the magazine and insert the case from the bottom up into the bolt face. Until you have developed a crush fit you need to neck size only or partial neck size if you only have a full length die. Once you develop a slight crush fit then you can push the shoulder back .001" to try and preserve contact at the shoulder without the crush fit.

Out of curiosity, what is the measurement on the shoulder of new unfired cases and what kind of brass?
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Old 09-04-2008, 08:49 AM
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Re: Help on shoulder bump for 7MM Rem Mag

It is Winchester brass, and I don't have any new, unfired ones. All are twice fired, and the one's that do measure the longest I can feel a slight pressure when chambering, so I guess I should size them all back to the 2.118"?
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Old 09-04-2008, 09:21 AM
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Re: Help on shoulder bump for 7MM Rem Mag

Quote:
Originally Posted by txsendero View Post
It is Winchester brass, and I don't have any new, unfired ones. All are twice fired, and the one's that do measure the longest I can feel a slight pressure when chambering, so I guess I should size them all back to the 2.118"?
That is reasonable. Before you start resizing, take a decapping punch and simply remove the primers from some of your fired cases, then measure them again. It's quite easy for a spent primer to affect your measurements by a thou or 2.

On my 7RM, I try to push the shoulder back .002" from crush fit.

AJ
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Old 09-04-2008, 10:51 AM
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Re: Help on shoulder bump for 7MM Rem Mag

Thanks AJ, all the one's I have measured have already been deprimed ( I removed the expander ball and let out the decapping pin so that the case and neck/shoulder wouldn't be sized while being deprimed).
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Old 09-04-2008, 08:25 PM
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Re: Help on shoulder bump for 7MM Rem Mag

Quote:
Originally Posted by txsendero View Post
It is Winchester brass, and I don't have any new, unfired ones. All are twice fired, and the one's that do measure the longest I can feel a slight pressure when chambering, so I guess I should size them all back to the 2.118"?
If you get a slight crush fit on the longest which would be 2.121" then that is the measurement you should try to duplicate. When you size the case the die will contact the case body to resize it and push the shorter ones out to match and all the shoulders should have the same measurement. If you push it back to the 2.118" mark then you will be working the brass more than is necessary which over time will lead to more thinning at the pressure ring and eventual case head separation.
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  #7  
Old 09-05-2008, 08:01 AM
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Re: Help on shoulder bump for 7MM Rem Mag

Thanks Woods. Like I said, always a lot of great info on this site.
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