Hello Coyote 1949, You will need an all metal, 100 % metal, empty 12 gauge shotgun shell from the 1890s or early 1900s to use to check your chamber. The dimensions of the all metal shotgun case that your gun was designed to use are slightly different then the dimensions of a modern 12 gauge shotgun. The length of the chamber of your W W Greener will also be very very different from what our modern 12 gauge shotgun chambers are. Good Shooting To You, Jent......As others have told you here, you have a 12 gauge shotgun according to the barrel marks that you have reported and posted here.
