So you have a pre-Sweet Sixteen- 1930 or earlier model 3 or 5 shot? Are you absolutely sure it wasn't convert to 2 3/4" alot of them were? And also unfortunate you can't just get newer 2 3/4" barrel and put it on the one you have !
As too the 2 1/2 " shells it may or may not function the action because the A5's are inertial actions meaning you have to have enough pressure to work it properly! Mine likes heavy field loads best!
Love the A5's have three of them now, Light 12, Mag 12 and the Sweet!