Here is the definition from MidwayUSA GunTec

A type of shotgun barrel constriction used on a barrel that either does not have a choke, or that has had the choke or choke thread portion cut off. Using an expanding reamer, a small area of the bore is recessed about one to two inches back from the muzzle. When fired, the shot pellets will tend to bunch up at area of slightly larger diameter, be constricted again the remaining bore of the original diameter and pattern in a slightly tighter formation.
http://www.midwayusa.com/guntecdictionary.exe/showterm?TermID=2956

A jug choke right from the maker is very unusual, but not unheard of.

Pete

http://www.damascus-barrels.com/Movie.html