If it is not stamped on the barrel or you do not trust the barrel markings, the only reliable way is to have a gunsmith make a casting of the chamber.
45 caliber Machine Gun and 50 caliber machine gun
Blue Book of Gun Values, on line auction.
gun shop, gun show
the 12 guage is a caliber not a gun
gun shows, gun shops, want ads
List the maker, caliber, type and serial number.
Caliber and bore are used interchangeably nowadays. There are technical differences you can find in the dictionary if you wish.
You can try e-gunparts.com and gun shows.
Gun shop, gun show, library, internet search
Want ads, e-gunparts.com, gun shop, gun show.
a .45 caliber.
Only way to find out is to look at old gun catalogs from the 60-70's