BrentD; Using Greenhill's formula for twist as L=150D²/T then you should be good for a bullet of around 1.2" long, which would be quite heavy in .635dia I suspect. You might check with your local Library & see if they have a copy of Ned Robert's "The Muzzle Loading Caplock Rifle". I heve read this yrs ago, & while it mostly deals with rifles of around .45 cal as I recall, it did deal a lot more with conicals than round ball guns. Might find something useful in it.
Miller

Miller/TN
I Didn't Say Everything I Said, Yogi Berra