I'd rather cast my own. Too, I don't really understand how a .50 cal bullet would need expansion? I'd think .50 cal would be about big enough and a good flat point would transfer enough energy while assuring full penetration. Of course, I've never shot anything with the .50, yet, but paper, but I've shot game with hard cast .357 magnum bullets that didn't expand and they worked great! Didn't drop 'em in their tracks, but they left a good blood trail and didn't go far. I think, too, that a 250 grain .50, especially that expands, would lack penetration. I have confidence that a flat nose 360 grain minie will penetrate to get the job done.
That bullet sounds a little light for my gun, anyway, with its 1:24 twist. It doesn't like light conicals, prefers the heavy stuff. The Hornady 385 grain pill works really well in it, and minies from my new Lee 360 grain mold. I tried a .250 grain conical and weren't too good, figuring it'd be a lot better out of a 1:48 twist.