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When to trim 7MM Rem Mag brass?

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Unread 08-12-2010, 09:23 AM
Bronze Member
Join Date: Jan 2010
Posts: 68
When to trim 7MM Rem Mag brass?

So, it seems that my 7MM rem Mag brass is over the 2.500 length after just two shots. I am not loading these hot and am using winchester brass. It appears that they are just a tad over and will not fit in the guage. How much over can I go before I have to start trimming? when you trim, doi you go .010 under? Thanks
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Unread 08-12-2010, 09:54 AM
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Join Date: Aug 2009
Location: Carlisle, PA
Posts: 478
Re: When to trim 7MM Rem Mag brass?

I trim to the trim length in the Hornady manual (which is pretty much the same in all manuals) - 2.490". I never allow it to go over the SAAMI spec case length of 2.500". I check case length after the brass is cleaned and sized and before it is primed.

I control case stretch as much as I can by how I size the brass. I get 4, sometimes 5, reloads between trims if I FL size to bump the shoulders 0.001" to 0.002". If the brass is only getting two reloads per trim I expect it is being sized to bump the shoulders .005" or more. If the sizing die is set up per the manufacturer's instructions it will bump the shoulders on the brass back considerably more than is necessary in the typical chamber - usually .003" to .005" more than necessary.

Why is trimming important? Allowing case necks to be over length can cause serious pressure problems if the case neck is long enough that it gets jammed into the end of the neck region of the chamber at the throat and effectively crimps the bullet into the brass when the bolt cams closed. In that condition it takes much higher pressures to get the bullet moving much like it does when it is jammed into the lands.

That said, if you want to know how long your chamber neck really is, it may be a couple thou longer than the SAAMI spec, you can measure it. Sinclair sells a tool to do this, or you can easily make one if you have access to a lathe.

I doubt pushing case length (really the neck length) a 3 or 4 more thou is going to do anything useful and it can result in damage to at least the brass if it causes crimping and over spec. peak pressures.

"For a successful technology, reality must take precedence over public relations, for nature cannot be fooled." R. Feynman. (Last sentence of the Feynman appendix to the Space Shuttle Challenger Report.)
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Unread 08-12-2010, 09:21 PM
DKA DKA is offline
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Join Date: Sep 2009
Location: Lumberton, NC
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Re: When to trim 7MM Rem Mag brass?

I check each time that I reload. Some will need trimming more often than others. Always trim to book length.
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Unread 08-12-2010, 10:28 PM
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Join Date: Mar 2010
Location: Spring, TX
Posts: 353
Re: When to trim 7MM Rem Mag brass?

Tres Monceret's article on this site would disagree with trimming brass to book specs. My personal opinion is the same. Spend the 8 bucks on the Sinclair gauge and trim to "your rifle" specs.
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Unread 08-13-2010, 04:09 PM
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Join Date: Oct 2007
Location: Mountians of SW NC, near Asheville
Posts: 1,595
Re: When to trim 7MM Rem Mag brass?

When to trim 7MM Rem Mag brass? After they exceed max case length seems good, no matter how few times they've been fired. ???

I have the Sinclair chamber length plugs, find most chambers are from 20 to 35 thou longer than max book case lengths. But, trimming is so easy and allowing cases to get longer means nothing so....

Excessive setback of the shoulders during FL sizing is what causes rifle cases to grow fast and eventually seperate at the head. It's not the pressure, not the shape of the shoulder, not the rim or belt, etc., just the improper way we excessively FL size and move the shoulders too much.
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