JRH,
Very nice drilling.
Doing a CHAMBER cast is very easy if you want to do one yourself. Order some Cerrosafe from Brownells and follow the instructions.
It does not sound like the gunsmith did a CAST it sounds like he SLUGGED the bore. I am surprised that he gave you such a broad range of bore diameter. Once slugged (also very easy to do yourself) you can measure the slug with calipers and usually tell exactly what the bore diameter is to thousandths of an inch.
Bore diameter is NOT the right way to determine the caliber the gun is CHAMBERED for. My double rifle is an Austrian .450x3 1/4" BPE. When I slugged the barrel I found it was .452" instead of the nominal .458 for most of these rifles. In my case the bore being undersized was a good thing as it allows me to use lead bullets sold just about everywhere for the Casull and Colt pistols.
I wrote the above to show that you need to do some more research/investigation before you attempt to shoot the rifle barrel. Don't take this wrong, I don't want to see your fingers separated from your hand.
