1:48 is a bullet twist. The other factor is rifling depth - you want shallow rifling for minies to be able to engage the rifling and seal - typically if the rifle has the 1:48, it will have shallow rifling. In a 1:66, or roundball, barrel, the rifling is usually deeper since the patch is doing the sealing work. As to barrel length, T/C made some real short barrel hunting rifles designed for bullet shooting.
All this info is worth what you paid - I've never contaminated any of my blackpowder guns with either bullets or Pyrodex - all patched round ball over black powder. Not a snob (well, not much) just never done it. And don't plan to...