http://www.mountainmolds.com/

Goto: Mountain man molds and play with a .550 bullet with a .540 base, change the weight and nose style......and you will see how weight and shape change the twist needed..............it is shown at the bottom.

Looks like a 400 grainer is about best in a 1-48 twist...........if you are going to shoot slugs, get a different barrel. 1in36 or faster, or be prepared to use a lot of powder and wading to get those big slugs to spin. And did I mention the kick.............[:'(]

Good Luck,
Bruce