Hi guys, new to this forum, but not new to guns, etc. Converting range pick-ups for my own AAC SD, I have had the same issues, quickly discovering that certain brands of 223 cases are too thick in the neck area AFTER the case has been formed. I've been using range pick-ups in 223, 204 ruger and 222 rem, trimming and resizing them TWICE through my Lee dies. I "slugged" my chamber and measured it carefully against the SAAMI drawings and found it to be almost prefectly accurate in size. While neck THICKNESS is a good measuring point, I found that neck DIAMETER AFTER forming is a better indicator of size. My rifle will tolerate 0.334 neck dia. with a bullet seated. Dia. larger than that (0.3345 and higher) will NOT chamber fully or at all. A 0.334 LOADED size equates to 0.332 MAX with a genuine 0.308 dia. bullet. I'd already made up some cases and run-in loads prior to receiving the rifle, and was bemused to find only half would chamber. My cure was to purchase the neck turning attachment for my Lyman Accu trimmer and turn the necks to 0.329 AFTER sizing from remade brass. Each and EVERY case gets measured at the neck 0.080 thou down and any oversize ones are turned. My working limit is 0.332 without a bullet seated. So far, I have had zero issues with chambering, closing the bolt, firing, extraction, etc.
Current case brands: Lake City NATO, Federal NATO & commercial (222 rem and 223), PPU, Winchester (222 rem, 223 and 204R), Hornady (204 Ruger), ADI, RP, Geco, WCC NATO, and several other military cases. MOST form fine and DO NOT require neck turning. I have also found that the more the case has been fired PRIOR to converting to Blackout, the LESS likely it will need turning. 222 Rem brass needs anealling because the die is expanding the neck, not swaging down the case body to size. Usually, 1 in 3 - 4 will neck split if I don't aneall them, but when formed, they work just fine. Just my $0.02
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