I have an Accura V2 LR .45. A bullet sized to .4515 fits nicely. The first barrel was tight at the end and looser as it went down. I sent it back and it is better. Not the "Most Accurate Muzzleloader" as advertised, but acceptable.
I like heavy for caliber bullets, so I shoot .45-70 (.458) bullets) sized down to fit. I bought a couple of Lee sizers in .454, and .452 and then size it just right in my adjustable Swinglock die. With soft bullets, I think I could probably just do one pass with the Swinglock.
I've never tried Powerbelts.