Sounds like a Vena Contracta.
On the 15th of June 1893 Horatio F. Phillips, sometimes gun editor of THE FIELD, was granted a patent in which, The gun barrels of shot guns are made of diminishing internal diameter for the cartridge chamber to the muzzle
The Encyclopdia of Sport said it was of a form known in the scientific gun-making world as a Vena Contracta this joins the 18-gauge barrel to the 12-gauge cartridge chamber. and claimed It was introduced by Joseph Lang and Son in 1893.
Contemporary Joseph Lang adverting featured a cross section of the barrel and said For Special Trials of the Vena Contracta made by the Editor of the Field, see Field Oct 10 and 17th 1893.
Most extant examples are engraved with the Lang name and sometimes feature two-inch chambers. Typically they are built on Webley flat back sidelock actions which incorporate the Rogers cocking system of 1881 and the Webley and Brain patent screw grip (dolls head) of 1882. Most have a tre-foil motif engraved to the fences.
Douglas Tate
Editor at Large
SHOOTING SPORTSMAN
The Magazine of Wingshooting & Fine Guns.
