Possibilities are:
1. You have a 20 gauge gun with tight chambers and slightly smaller bore for shooting fibre wads from thin walled brass shells (similar to the 13 bore 12 gauge guns)
(are measuring the muzzle or the bore, if muzzle you may be getting a way off number because you have a tight choke?)(also how accurate is you measuring tool, you may not be accurate on the .680)
2. You have a 24 ga gun with oversized bores and chamber ?
3. You have a custom gun of a very odd gauge like 22 etc.?
If you have very tight chambers with an undersized bore your best course is to cast the chamber and have RMC make you shells.
Before you know the answer you really need to post the barrel flat and barrel stamps and you really need someone with a proper bore gauge to measure the chamber, forcing cone, bore, and choke.
Last edited by old colonel; 01/23/17 11:10 PM.
