It isn't clear, but I think you're talking about bare, lubricated, lead (Pb) conicals, as opposed to the little jacketed pistol bullets in a plastic sabot that so many use. They do seem to be quite popular. I don't like 'em particularly, they are stinkers to load (take lots of pressure to get 'em home), and I don't see the point anyway. Ah, well...whatever strokes yer, um, ah...Well, anyway, I shoot roundball because my rifle has a twist rate of 1:70", and I'd be a fool to try anything else.
To shoot a conical, which is longer and heavier than a round ball, you need a faster twist. The T/C rifles have popularized the 1:48" twist; it was also used by Hawken, but he had only one rifling machine, which would tend to indicate that it was a matter of using what he had. At any rate, the 1:48" twist seems to work pretty well for bullets as long as a Maxi-Ball, but I can't say anything about longer ones. It also seems to stabilize round balls acceptably. Drat it all, anyway!...I used to have a chart (a Nomograph, actually) that plotted out twist rates for various length projectiles driven at various velocities. That would have made it too easy! The thing came from Gun Digest 'way back in the '50s or '60s...you might be able to get one from them. Many of the modern muzzleloaders are made with quite fast twists, in the neighbourhood of 1:32", but I have no idea where you would start in finding either the bullet or a mould for them. Unless you're using some special concoction for powder, that bullet better have BIG ol' lube grooves in it, too, to keep the fouling soft.
