Just learning to reload. Starting with 9mm Luger casings fired in my P226, and for reuse in same weapon.

Lyman 49th ed. says to trim to 0.754, Lee's 2nd edition Modern Reloading also gives same figure.

When I put a dial caliper to my brass, they range from 0.753 to 0.742 in case length. The average length over about 200 casings is in the 0.745 area.

If I understand correctly, my Lee case trimmer will not remove anything from these casings.

If I were to assemble cartridges with these casings I believe there would be excess headspace which I've read should be avoided. Am I wrong? Why did the fire OK previously?

Newcomer kind of confused here.
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