Hello Perry,The term "chamberless 12" is actually a bit of a misnomer in that the barrels actually had a chamber with very thin forcing cones,which was made for a thin brass 12 bore case that could be up to 3"long and an 11 bore wad was used to fit the case and the oversized bore which was also 11 bore. These guns were basically built on a 10 bore action and barrels.This gun has 3 sets of proof marks.The first is 12 bore black powder,the second is 12 bore 3" nitro,and the last was done in 1962 when the gun was rechambered to 3 1/2" 10 and nitro proofed to 4 Tons /sq. in. with a bore diameter of .765 which it has today.It has 45 thou of choke in both barrels. Hope that all makes sense...
