On the gauge markings note that prior to 1887 neither the nominal chamber size Nor the in between gauge marks were stamped on a bbl at proof. Thus a bbl stamped 11 had a bore that took a .761" gage to 9" from breech but would not take a .775" gage. This size bore was most often found on gun chamberd 10ga but could be found on a 12ga chambered one, perticularly if all brass sheels were to be used. The proof house had the gages in all the whole numbered gauge sizes & stamped the bbl according to the largest one which would enter the breech to the desired depth. Prior to the general acceptance of choke boring they had been gaged from the muzzle, a hold over from muzzle loading days.
