What Triple Se7en said is true with the fast twist you need a longer bullet which means a heavier one. A friend who owns a 1:20 twist White Mountain Carbine had real good results with anything above 420g with that rifle.

I don't have bullet restrictions so I have not tried any of the heavy conicals yet but soon will be in a fast .54 rifle.

Cayugad has one of those rifles and loves to shoot it so I would follow his advice for the Black Diamond.