I haven’t had one in a Contender, but have in a Hawkeye. I’ve strongly considered building one on a GP100 as well. H110/W296 has been my go-to powder for the 256, just like any other magnum rimmed cartridge round. With the light 75’s, you won’t be heavy enough, I don’t believe, to warrant lil’ gun, so in your shoes, I’d keep my focus on H110/W296.