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Is this normal?
In the process of forming my 30BR brass from Lapua 6mmBR, I had trimmed all of my brass to 1.500". After firing them for the first time, they now measure 1.503", 0.003" longer.
Is the 0.003" growth normal?
How much room do I need to leave between the case mouth and the end of the chamber?
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Howdy. I guess my first question is: are the primers still in the fired cases? This can skew the dimensions a thou. or three. Even if the primer pocket was deepened during the prep process, the primer may be slightly above the case head after firing. It seems that the cases need to fired at least once before the primer pocket settles down...this may also have to do with the case head squaring itself to the boltface as the case is fired the first time. Add a few of these subtle little things together and it's not surprising that we can sometimes see a change of a couple thou. when we first fire a case.
Dimensional case changes are also a product of how well the virgin case fits the chamber. For example, an Ackley-style case will shorten when fired the first time as the shoulder and case body blow out to fill the chamber.
If it was me, I'd just measure with the primers removed, trim back to 1.500 and go from there. Assuming you have an 1.520 long 'Robinett' chamber, you could trim when the cases get to 1.510 and be in good shape.
You may also want to try checking the o.a.l. on a deprimed fired case both before and after you size it. If the o.a.l. increases a bit after sizing, it's an indication that you may be sizing the shoulder diameter a bit more than needed.
Hope this helps. -Al