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Size your cases to around 0.004" under your rifle chamber headspace and you should be good. After firing your brass should come out all roughly the same cartridge headspace length. I found that measuring before and after firing and comparing the two cleared up a lot of mystery about headspace. The wild card in the M1A / M14 world is the effect of the forcefull ejection on the case, every rifle is different and will subject each ejected case to it's own version of torture.
John
John