I tumble PC'd some bullets and shot them today using a load that has delivered great accuracy with a non-PC bullet (frequently well under 1MOA w/ fliers occasionally).
257 Roberts AI, 257312 GC, sized, 15grs 2400
3 5-shot groups at 100yds
#1 - 7/8" - very satisfying group all inside a 1" black circle.
#2 and #3 were baseball sized groups (~3") with no rhyme or reason to them--just big and scattered here and there, up and down
Velocity was 1950fps--essentially the same that I get with non-PC.
No leading
I used 2:1 COWW to soft lead. The non-PC bullets were water quenched and culled (maybe 20% of the outliers). PC bullets were NOT water quenched and ruthlessly culled and also sorted by weight--should be better in my mind.
I used HF red PC, tumbled on and baked on their sides in a wire rack.
I'm puzzled why group #1 would be gorgeous and the other two would be completely different. No elevation or windage diff in them either.