I carried a .243 with 100 partitions once, but only because my main .308 rifle had problems. The smallest I've carried on purpose was a .257 Roberts with 120 NP at 2800fps. I agree with many here who say 6.5mm is a sensible minimum bore size. Bullet construction becomes critical with the little ones, so, Partitions, A frames, TSX, and bonded cores only please.