re: 10-shot groups to test rifle/load accuracy.

Well the M14's are known to beat up brass. It's how the gas system works, basically starts the extraction process while the case is still obturated in the chamber. That puts a lot of extra strain on the base of the brass. Rule of thumb is to toss the brass after 4 uses with the M1A/M14 and that's with mil-spec brass which is tougher and little thicker in the web and neck area. I was using PMC Bronze to get the initial zero but used some leftover Black Hills match to see what would happen. Both are supposed to be somewhat tougher than regular commercial brass but probably not as tough as Lake City M80. Problem is M80 is steel jacketed and I don't plan on running it.