You beat me to it, Ted.
As for only the gauge and not the chamber length . . . well, in the case of the specified 12ga, they make them from 2" up to 3 1/2". If you could fit a 3 1/2" shell in a 2" chamber, or even a 2 1/2" chamber, I think you'd get a rather unpleasant surprise when you pulled the trigger. That's too much additional case length . . . not to mention the fact that the 3 1/2" 12ga has a much higher proof than the 2 3/4" or 3" guns. And even higher still than the 2" or 2 1/2" guns.
Most loaders are set up for 2 3/4" hulls, which are readily available (and of very good quality) in this country. Given the price of factory 2 1/2" shells, it makes a whole lot of economic sense to reload 2 3/4" hulls to appropriately low pressures, especially if you're going to shoot a 2 1/2" gun a lot.
And we now know why CA is in dire economic straits. I thought it was just the politicians, but it appears to have short-circuited Wonko too. Must be something in the water.
