Surprisingly, the most accurate powder I've used for 50-55 grainers has been IMR-4198. A little velocity is lost, but damn it puts the bullets close together.

I have some 77 grain SMK loads using Norma 203B (RL15) I want to try soon. H322 has been so-so and CFE223 is a little better, but I feel like something's still on the table.

H322 has done very good with 69 grain SMK loads.