Agree with Miller that it sounds like a strange version of jug choking. However, it does seem to fit the description for barrels marked CHOKE rather than NOT FOR BALL under the proof rules of 1868:

" ... barrels, the bore of which was enlarged immediately behind the muzzle, but except for such enlargement, and also except for the chamber or recess containing the cartridge, the bore was not in any part of it larger than at the muzzle."