Thread: O A Length
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Old January 24, 2012, 12:14 PM   #3
Join Date: March 4, 2005
Location: Ohio
Posts: 13,714

The way SAAMI specs on cases work is they specify a maximum with a minus tolerance. The tolerance is different for different cases, but for most rifle cases it is -0.020". The trim-to number you see in some manuals is simply in the middle of that tolerance to give you the most room for error either side of it.

SAAMI allows free access to their specs published in 1992. A number of cartridges have been added since then, but the common standard rounds are in the old information. You find your cartridge in the lists in the first half of the document and those drawing have the cartridge on top and the chamber below. The second half of the document includes pressure and velocity test barrel information.

All that said, for some cartridges you can trim below SAAMI spec. A lot of service rifle match shooters used to trim to -.035 to -.040 below SAAMI maximum when a case was new just so they could trim once and never trim again before they wore them cases out, which happens faster in a semi-auto gas gun than in a bolt gun. That practice is not best for consistent neck tension and is not useful in cases that will be crimped, but for non-crimped match loads for offhand and the sitting and prone rapid phases, the accuracy is fine.
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