Originally Posted by justgoto
You'll be fine. I've bought plenty of brass new that was shorter than the trim length; it'll grow.
Before you measure your case, run it through your full length sizer. The measurement will be different after that. Then you turn or trim, beburr flash holes and primer pockets.
The amount your case will lengthen on each shot varies with each lot of brass. I can go several firing between actually running my case in my trimmer and just using a chamfering and reaming tool.
If your brass is chambering in your rifle and isn't growing a second lip around the mouth, I wouldn't worry about it. If you jam the mouth into your chamber too far, the entire neck can come off. I have seen this on a military 30-06 with surpluss ammo. I measured the ammo for the guy that owned that gun. I think the case jammed almost .020 into the chamber. The bolt closed very tight. I wouldn't have pulled the trigger to begin with.