The outside diameter of the 12A brass shell is the same as the standard 12 gauge paper shell. The 12B brass shell is smaller than the 12A. Guns chambered for the 12A will handle 'modern' paper and plastic shells. There's a little more to it, overall, but that is it in a nutshell.
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GMC(SW)-USN, Retired
'Earnest Will'
'Desert Shield'
'Desert Storm'
'Southern Watch'
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