Interesting. I don't think I've ever noticed more than 0.1 grains difference in two SMK's unless you went to different lots.
Slamfire brings up a good point. The reason the military teams would toss a bullet for a small weight difference is concern it might indicate a lead shaving or something that wasn't symmetrical around the center of mass. Not likely, but could happen, and at 600 a bullet wobbling from mass asymmetry would, indeed, be less accurate than normal. If you don't have a bullet spinner or other method of determining mass symmetry it becomes a form of precaution to do that, just in case.
If you want to see if your bullets have an issue like that, just segregate out the extra heavy and light ones and load and shoot them separately to see if your groups change size. Most likely they won't, but what the heck. This is a hobby, after all and you may as well have fun trying things out.
I think it was ±1.5 grains; i.e., 174.5 grains +0/-3 grains. Saw the spec somewhere, but don't recall where off the top of my head. Despite that range, when Frankford Arsenal rolled the NM ammo, they used set aside lots of those bullets that had proven extra accurate, so they were probably more consistent in weight than that 3 grain spread in any given year. I heard that at some point after LC took over rolling NM ammunition, though, their management put a stop to that extra testing and sorting, so mixed, non-select lots began to be used and NM ammunition was never as good again. Don't know what year though.
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Last edited by Unclenick; November 9, 2011 at 04:20 PM.