While wondering what gauge other guns may be, it won't matter when comparing to yours...
You
cannot guess... A 16 gauge would look nearly the same as a 12...
What you need is to measure the bore... at muzzle... Considering there may/will be a choked diameter...
Here in this link is a chart with bore diameters of each gauge along with the dimensions to
subtract from "true bore" or cylinder choke (no choke at all)...
http://www.colonialarms.com/chokespecs.html
That will tell you what gauge your gun is...
Being quite old, you cannot assume it to be a 3 inch shell chamber nor even a 2 3/4 inch chamber... A QUALIFIED gunsmith will have a set of chamber gauges in his tool assortment...
Basically will do what is shown in the pic with this link...
http://www.csmcspecials.com/Handy_Mo...ge_p/t0321.htm
If you have a 2 9/16ths chamber, a 2 3/4
unfired round
will fit but wouldn't be safe to fire it...
You need both of these dimensions before trying toload and fire this firearm...
Brent