I recently bought an 1885 10 bore from a seller who told me that it has 3" chambers according to his chamber gauge. I don't have a gauge, but an unfired 3.5" 10 bore brass shell leaves 1/4" protruding from the breech, suggesting that the chambers may in fact be 3.25". This test worked when I tried it on a 2.5" 12 bore: a 2.75" brass shell only goes in 2.5".

Is this a valid test of chamber length?