Bore size isnt all that relevant; do a chamber cast and put a picture up of the proof marks and get someone to measure it properly with a shotgun bore micrometer or plug gauge 9" from the breech
Is there a visible end to the chambers or can you see a forcing cone? Chamber-less guns were experimented with usually designed for use with brass cases.
Bore size is almost irrelevant, ive seen 12 bores which were 11 in the bore, and 12 bores that were 16 in the bore. You could just as well have a 14 bore
The diameter at the breech should be around .812 for a 12 bore and about .775 for a 14 bore. Make allowances for wear and manufacturing error and such.
