Brad, let me put some perspective on it: The 6/4 XCB bullets you and I have been shooting test at exactly 24 BHN.
They be water-quenched, no doubt about it, I didn't even have to ask Fiver. Not going to do much HV work without heat treating or coppering. Reason being that when you add enough antimony etc. to make them tough enough to handle the land torque, the metal is so brittle it just crumbles off the leading side of the land and leaks on the trailing side, poof goes your lube and your accuracy. Basically I think we need to work on an alloy near if not equal to his alloy, treat it as a base, and do exactly what Rick did with his 357 Max and experiment with heat treating it to different levels to achieve the characteristics that our rifles tell us they want.
We know there needs to be some tin in there to control sloughing of metal, the question I still have is how much. I know 6% Sb and 4% Sn worked like a champ bumping near 2500 fps in a ten-twist, with less fouling (basically none visible at all) in the corners of the lands than I've ever seen before in anything above 2400 fps. 5/3 might be fine, and can be made harder via heat treat than by water-quenching from the mould. 4/3 or 4/2 might also be fine at maximum quench. 5/3 might be perfect at a medium heat treat. Bullet design will likely have a lot to do with which works best, but at this point the more I do with richer alloy the less I know. Ask me about 50/50 or straight WW with various amounts of tin and I don't look quite so dumb.