The bolster is the large area behind the barrels where you have the pitting and the firing pins emerge. You should measure across the bolster in line with the firing pins which should be the widest point.
You should measure at the muzzle to determine gauge.
I would think that serious blowback would occur if you fired a 12 Ga in a 10 Ga chamber. I have both 12 Ga and 10 Ga antique brass shells and the 12 Ga slips inside the 10 Ga.
1882 identification
Re: 1882 identification
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[/URL][/img]Re: 1882 identification
2.353 for the bolster
0.722 for the bore, other barrel was 0.710
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0.722 for the bore, other barrel was 0.710
[/URL][/img][/URL][/img]Re: 1882 identification
Looks lik a 12Ga to me. Fairly tight choke.