Chris, I'm referring to just the chamber end of the barrel (where the ctg. goes). Generally speaking there are 3 types of ctg. body shapes: straight (cylindrical) tapered (tapered cylinder, from base to neck where bullet seats, naturally), and bottleneck where the back of the case is measurably wider than the bore and there are either two tapers with a shoulder radius connecting them, or a tapered or straight body connected to a straight neck by a radiused shoulder. That last type looks a little like a beer (or wine if you prefer) bottle and is called "bottle-necked". Yours could be any of the above; usually you can see the shoulder in a bottle-necked chamber, the others can only be differentiated by measurement or by eyeballing a cast of the chamber. Casts, as Raimey points out, aren't rocket science. Just be sure you use a material that releases from the chamber metal easily when it cools.
The cartridge which I mentioned above is called the 10.3x65R (the older thin-rimmed version) or 10.3x65R Baenziger (the newer thicker rimmed version). These are apparently rifle cartridges that are the same dimensions as the 2 1/2" (NOT 2") version of the present .410 shotgun shell. Which came first, shotgun or rifle, I do not know. But you might try fitting a 2 1/2" .410 shell into your chamber (DON'T cock those hammers) and if it is a fairly snug fit, then you might have a clue as to caliber. HOWEVER, you would still need to know which version of the 10.3 it is. And that makes a difference because if you make or find the wrong shell it either won't fit (Baenziger in thin-rim 10.3) or will have excessive headspace (thin-rim 10.3 in Baenziger chamber). Aren't old guns fun?????
If you aren't familiar with precision measurements, I'd get hold of a gunsmith who has the instruments and Cerro Safe for a chamber cast and have him tell you exactly what you've got (The gun looks to me as if it is worth doing this right). You will get the $$ back when you sell it by being able to tell the buyer exactly what they're getting (I think). If you want to ever shoot the gun, you will definitely need the services of a gunsmith to identify the rifle and shotgun chambering precisely and check for safety.
Have fun.
Mike Armstrong
The two 10.3x65R ctgs. are described in "Cartridges of the World," a book that is widely available in big box bookstores. You could probably go to Borders or Barnes and Noble and have a squint at the page for free. It's under "Metric Sporting Cartridges."
