Stan,
One of my Einstecklaufs is regulated that way( with an eccentric on the muzzle of the EL which has a slot across the front, and is turned by a screwdriver from the muzzle of the shotgun to regulate the EL). You could also put a collar on the muzzle end, with 4 small set screws and regulate it like a 4 jaw chuck. A lathe is a necessity of life( a mill too).
Mike