The standard .58 minnie casts .575" in pure lead. In WW or other alloyed metals, look for .001" to .003" larger from the same moulds. The skirts of normal minnies are too thin for heavy loads of any type as they'll 'blow' coming out the muzzle.

48" is an OK twist for short, 500gr. to 580gr. bullets. The longer the bullet, the faster the twist is required - normal stuff. My .58 Hawken shot well with a 480gr. modified Old Style minnie with a modified and shortened thick skirt using 140gr. 2F. The heavier 570gr. Lyman Mould, modified (thickened skirt) to 610gr. shot less well and of course, kicked pretty hard with the same 140gr. charge, but did group reasonably well at 100 yards. That was all from a 60" twist Bill Large barrel, custom rifle. Lighter loads with the heavy bullet would cause slight tipping going through the paper and the heavy really didn't make nice round holes at 100 even with the heavier charges - slight tipping was evident, although grouping was usually into 2 1/2". The shorter bullets shot tighter - going 1 1/2" & made nice round holes. 480gr. is VERY short and light for a .58 cal bullet.

I think any desire to shoot heavy, 700gr. to 900gr. longer bullets should be attempted with a much faster twist, to allow stabiltiy after upsettage in an animal. Twists on the edge of being unstable cause bullet tumbling, and verring off course, bouncing off bones instead of smashing them. This, we've seen using 410gr., 370gr. slugs from 48" twists in .54cal. and .50 cal. muzzleloaders. On lioght game like deer, this usually doens't happen, but on heavy game like Moose and Elk, it does. Slow twists and long slugs don't work well together - for us.

The larger the bore, the slower the twist can be - IF bullet length is held short. I'd still go with a 28" to 36" twist in a breechloader. It will shoot all bullet weights you are likely to use - smokeless or BP. I'd hold groove depth to from .005" -.008" max. My opinion, with facts and experiences.