The issue was indeed trying to use brass formed for an old barrel! I picked up 200 Starline .223 cases, and formed up 100 of them.
Shot last match on Sunday with no problems, and worked like a charm.
Glad it wasn't the shellholder/die issue that I thought it might be.
Thanks for the advice, gents!
Finally got this set of dies and barrel figured out. New Starline cases, formed up nicely. Got it running, and shot a UAS 39 (didn't listen to spotter), and a UASHS 33 with no settings.
123 Hornady FMJ, 21.5 gr AA2200, CCI 400s. Range tested and found a better load - 24.5 grs H335. Looking forward to trying that out next match.