In the mid 1880's, the Brits ran twists for round balls as quickly as 36" - however they didn't work well, of course, especially as heavy loads were as innaccurate as a smoothbore due to slippage. Again, mid 1800's, the twist in the 3-band ML Enfield rifles was 1 in 78" for the 1851 and by the 2-band 1858, the twist had quickened to 1 in 48" for the conicals of the day- the 560gr. Pritchard or Pritchart Minnie style conical, with the square base, not pointed like the American 505gr. version.

I think I'd be most likely to try round balls first, but at 72", it will probably shoot a short conical just fine.

Conicals really didn't catch on well in muzzleloaders for dangerous game, due to them having to be pure lead. Pure lead did not penetrate well on dangerous game, thus the more prolific use of round balls, even though a 'conical mould' was generally "thrown into the gun case" with the rifle. Once ctg. cases became the 'in deal', conicals of hardened lead could be used and thus "took over" from the round balls of the day. Here, I will note, that hardened round balls could be used, as the cloth patch 'took' the rifling for accuracy. A conical had to expand into the rifling to shoot well, thus had to be soft.

Sorry if this is a bit disjointed - that's how I feel right now.

The moulds being much more cost effective - especially as Jeff Tanner of the UK make splendid moulds to fit standard handles.

Too - a good black powder lube will be necessary - Lyman's Black Powder Gold, or Steve Garbe's, SPG is also excellent.

Barring the availability of a good BP lube, try beeswax/Vaseline - 55:45 mix - or 60:40 for a harder mix that is still malleable. More beeswax makes the lube harder for higher temperatures.

All of these lubes work well with smokeless powder cast bullet loads as well - right to 2,000fps with .45 cal. or .50 - as I tested them.