I've been pushing 180 gr 308 cast, powder coated #315 bullets at anywhere from 2100-2500 fps with no leading. Alloy had maybe a quarter of the Tin you propose, and about half the Antimony. For my 45 pistols, both acp and Colt I use about half the sweetners as that mix. Using the same mix in 38-55 and 25-20 at up to 1600 with no gas checks. Hard isn't necessarily the answer.
