12" is ridiculously fast in my opinion, for a shoulder rifle. Even a .358 doesn't need a twist that fast, let alone anything larger.

Sounds like a barrel specifically made for 1,000gr. bullets from a .5 case. Normal for .5's is around 15" to 18" I think - for bullets to 800gr. I know a 24" twist will shoot amazingly well with 600gr. bullets.

The lighter bullets will shoot - that's a given, but you'll probably want to keep weight to 500gr. or over. It is possible a 450gr. bullet will also shoot.