I have a hand me down Lefever Shotgun that belonged to my grandfather but it has no marking on it that indicates gauge. I've contacted a Lefever collectors group but until I hear back from them I was curious if anyone had suggestions for definitively identifying antique shotgun gauges. I measured the inside diameter with a cloth tape and the best measurement I can come up with is 11/16 of an inch or 0.688 when converted to decimals. According to online charts I've looked at it seems like it would fall into the 16 or 20 gauge range but I don't trust my measurement obviously. Looking up the serial number of this gun only tells me that it was likely made in 1900 and the only other numbers I can readily see stamped on the gun are patent numbers. Any suggestions? Thanks