I note in the listing seller states the bores measure .652". A 16ga goes .662", a 17ga .649" & an 18ga is .637". This gun was likely proofed prior to the introduction of sub-gauges so there would have been no 18/1 at time of proof. Thus it could have been close to the 17ga dia at proof & been polished out a little over the years. As long as wall thickness is ample 16ga wads should work for shooting it. For a number of years I shot an old I Hollis percussion dbl marked 12 with 11ga wads as the bores measured about .750".
