Older shotguns and shotguns with a fixed choke can usually be threaded for removable choke tube systems. The thickness of the barrel wall is the determining factor. To safely thread the barrel a given thickness must remain after the threading in the barrel wall. Several types of threading are available. Without an accurate bore micrometer and an outside diameter micrometer it is impossible to determine whether a barrel can be safely threaded for a given thread style. With a removable choke system any shotgun becomes a more versatile weapon.