Barrels were cut off when the breach was ruined from shooting, or muzzles worn from the muzzle end from loading and cleaning...They were also refreshed (rifling) and rebored (recut and re-rifled), the gunsmith would make a new bullet mold as well....

No, sabots were not avaliable in the 1500s, with a round ball a sabot isn't necessary....There were a few inline flintlocks made in Europe, but they never caught on...