I went back and looked at my notes and found that it is 12 groove, and measured .4550/.4565. I just got back from the range and got a chance to shoot the harder bullits against the 1 in 20. I shot 4 groups with each and the harder alloy was clearly 1/3 smaller groups.I sized them down to.457. best I could do with soft was2" best with the hard was just out of 1" average was 1 1/4". Ill play with seating a little but over all Im happy. DR