I thought of that too, Robin, but I was worried about chamber damage and the wax getting stuck. I don't think chamber damage would be an issue, though--I know of several people who "wax" the outside of their firearms rather than oil them.

Thank you (again!) Dean. If my figures are right, a 12 gauge measures .729", a 13 ga.--.700, and a 14 ga.--.693, so a few hundredths less than .751" could be a 13 OR 14 gauge, since the 12 ga. brass was too large (.729) What diameter is a 12b that I read about in one of the posts? Is this a possibility for this gun?

Jack, you're confusing 'nominal bore diameters' with chamber diameters. The chamber diameter (slightly tapered) will (almost) always be larger than the bore diameter. The diameters you show are nominal bore diameters.

Ok, first, on the faceof it, I was wrong about trying a modern 12ga shell in an early Parker gun, Dean is correct, but also, so is Robin. At the time of this early gun there was brass shells marked A & B. According to The Parker Story, at about 1870 the A & B shells appeared on the market. The "A" shell was for use in chambers sized for paper shells of the time. Paper had thicker walls so also a larger outside diameter. Then, shells for earlier guns that were chambered originally for brass shells, were marked with a "B". TPS also states in an undated chart that 12ga had tapered chambers from .811 to .797. FWIW, I believe this size corresponds to size of modern 12ga shells. This size would also be the "B" size shell, as the "12A" was a larger diameter matching the overall paper shell of the time. Confused yet? So, I still think you can try a modern unfired 12ga shell to help make a determination of what you have. If the gun is a 13ga, then the chamber will be smaller, and the 12ga shell won't fit.

Yeah...I'm so confused at this point I think I will just wait until I can get a reliable gunsmith to take measurements for me and come up with a definitive answer. I'm just such an impatient person that I think I can get answers to everything "right now". I guess I'm going to have to learn to be a little more patient!

You're doing good, Jack. Sorry I type so slow, a lot of posts while I was hunting and pecking. Dean is simply stating that the muzzle size has nothing to do with gauge. It's all in the chambered shell.

Thanks so much for sticking with me, Dave! As I was telling Dean, as impatient as I am, I think I'm just going to have to wait and find a gunsmith who can make a gauge determination for me, even if it means waiting a while!

13 gauge-one last try!
 

One final try! I didn't make a mold, but I sanded down a plastic tube and then smoothed the surface. It now goes easily into the chamber where the shell fits and then slides very snugly through the entire length of the barrel. So snugly in fact, it takes a moderate amount of pressure on a dowel rod to push the tube through the barrel, although it loosens up at the very end (choke?). If I were to accurately measure the diameter of this snugly fitting tube,(I think the tube is pretty uniform in diameter) would this be pretty close to a true bore diameter? I don't have a caliper (that I can find!), but rough measurements with a ruler read +/- 18 cm. If anyone thinks this would be an accurate enough method, I will try and locate a good caliper and get a more accurate reading. Thanks for your patience and ALL your help!!!

First, bore is measured in inches. 18 millimeters is .709 inch. I don't know what that means, except it's probably smaller than a 12ga gun. Accurate bore and chamber dimensions will have to be taken with a gage that can measure in thousanths of an inch.

According to Circle Fly, 13ga. is .710" Since I can't find any published dimensions for Chambers, I'd be more inclined to rely on the info that is published, i.e., Bore dimensions.
I applaud your enthusiasm, and know well what you mean by being impatient, but using dowels, sanded PVC pipes, etc, to attempt to find something that is measured in .001" is akin to using binoculars, when a microscope is called for.

Why don't you let us know where you live, and perhaps you might find a PGCA member close by who owns a Chubb, or similar bore gauge. I have one, and live in N.Central CT.