I've recently acquired a Beretta over/under muzzleloader and was surprised to read the manual stating to remove and clean the breech plugs after every use. I've been out twice to put 2 rounds of clays through it (50 each) and afterward removed the plugs for cleaning, applied a dab of anti-seize, and reinstalled. They snug up and index easily and cleaning the barrels is a breeze.
This led me to reading a lot about breech plugs, both hand cut 18th century and modern removable plugs. Question is, since modern materials and CNC machining of parts for historical reproductions is acceptable, and modern threading equipment is so precise, why wouldn't a builder design the breech plugs to be removed easily for cleaning?