I've only had the 121 mold for a short time and have only cast a handful so far. I haven't shot any with BP yet, but did shoot the 121 (30:1) side by side with the 643 (WW alloy) in my 1886 - IMR 3031 in both. The 643 did shoot better than the 121, but not by much. My 121 mold does cast at .455 and I'll probably use it more in the Sharps and Ballard with BP. As you say, I do like the 643 in the '86 due to the crimp groove and nose shape.