I have a Lyman Trade rifle. Percussion in .50,.... I shot a friends Great Plains Rifle by Lymanand am now wishing I had bought the Great Plains. His is the 1:60 twist and is incredibly accurate with patched roundballs. The Trade rifle is 1:48 twist and is accurate with RB, but not like that Great Plains. The Trade Rifle shoots conicals better, though. I like the .50, but like Fg said, I'd get the .54 if hunting was my main goal.
Lyman does make a version of the Great Plains in a faster twist for conicals.
It's called the Great Plains Hunter and the twist is a fast 1:32. Also available in .50 or .54
I bought my Trade Rifle here....
http://www.dnrsports.com/acatalog/Lyman_Muzzleloaders.html