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Old September 11, 2012, 01:53 PM   #12
Unclenick
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Join Date: March 4, 2005
Location: Ohio
Posts: 10,221
I don't go over 2.015". No need to. A blown primer chews up the bolt face with gas cutting and really isn't worth risking to save a little trimming work. And if you have a chamber with a ball throat (taper from neck diameter, no freebore) it can wedge a long neck in, raising pressure. Keep in mind that chambering a round can set the shoulder back a few more thousandths (Hatcher measured 0.006", IIRC, on an Enfield chambering a .30-06, presumably done by someone who could work the bolt fast), so what fits when you drop it in the chamber empty might not do so during actual shooting.

SAAMI minimum case length is 1.995". Military minimum is 2.000". The usual 2.005" "trim-to" length given is simply half way between SAAMI minimum and maximum. Nothing magic about it. SAAMI is a manufacturing association, and manufacturers typical aim for the middle of a spec range to allow a maximum of wiggle room for error so they don't have to measure every cartridge case for length. No reason the handloader has to copy that.

Some M14/M1A competitors used to trim .308 Win to 1.975"-1.980" just so they were sure never to go over 2.015" and have to trim again during the life of the case. I heard it suggested that can eventually wear the chamber a little wider, increasing gas bypass during bullet jump, but I've never seen it demonstrated to make any difference during a barrel's normal lifetime. If someone else knows otherwise, please speak up.

Needless to say, you can't trim cases extra short and expect crimping dies to work normally with them. If you don't crimp normally (the guys using extra short trim didn't and neither do I) it doesn't affect you that way.
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