The original brass shot shells were designed to have the inside diameter of the shell the same as the bore size of the gun.  Imagine the chamber being reamed out so that when the brass shell is inserted the inside of the shell and the inside of the barrel line up to make a smooth tube for the shot and wads to travel through.  This is exactly what the 12B shot shell is, the ‘B’ indicates the inside diameter is the same as the Bore diameter (B represents Bore size).

It didn’t take long in the development process of ammunition to arrive at the point where paper replaced brass as the preferred container holding all the shot shell components together.  But the use of paper required a thicker paper wall to hold everything that the brass walls held and that caused a chamber size problem.  To get the same amount of powder, shot and wads into the paper shell, while keeping the inside diameter the same as the bore, required the outside diameter of the shell to increase.  When brass shells were made to fit the chambers of guns made to shoot the new paper shells, they were marked 12A to distinguish them from the smaller 12B shells.  

Because all brass shells maintained a common brass wall thickness and the outside diameter of the 12A shells was bigger than the 12B, it resulted in the 12A shells inside diameter being larger than the barrel bore diameter.  It seems that the ‘A’ designation of the 12A was meant to stand for the internal diameter being “above bore” size (‘A’ represents Above bore size).

By about 1875, more guns were being chambered for paper shells than brass because paper shells were cheaper to produce and if desired 12A shells could be used too.  At the time, it was known that 12A shells were harder hitting than 12 gauge paper shells and were preferred in some situations because of that feature.  The 12A shell could accommodate more powder and shot because the thin brass wall thickness allowed the 12A shells to have more volume inside the shell than either the 12B or the 12 gauge paper shell.   In fact the 12A shells used a wad size two sizes larger than the bore, resulting in a harder hitting and deeper penetrating shot shell.

The result of all this is that the popular 12 gauge paper shell and the 12A brass shell were the same chamber size and the standard in almost all 12 gauge guns seen today; but once in a while a smaller 12B chambered gun comes along and confuses the casual observer.   

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