Actually one of the most accurate bullet I've ever shot out of my .45 cal Hawken was a 370 grain lead bullets that was cast from an 11mm mold (.443"), paper patched and final sized to .447". That's a far cry from the .451" -.454" bore diameters of the average .45 cal muzzy.
IMO most muzzleloader barrels that are "shot out" is simply a lack of maintenance by the owner an the bore/rifling is badly leaded. As of yet I have no opinion as to the effect of poor maintenance on a heavy diet of saboted bullets in regard to sabots fouling the rifling.