2-piper, I do know for a fact that an opened 12 gauge, old style AA case is the same length as a Gamebore "Super Game" hull which states on the box that it is 67.5 mm and safe for use in 2 1/2" chambered English guns.
As I recall, the Gamebore kicked quite a bit more.
This was my first hint that paying $10 a box for special ammo is not always all it is cracked up to be.