The chambers are more than likly 2-1/2", shells are available from Ely, cost around $7.00 a box light and heavy Do not shoot shells that have plastic wads as they will buldge your barresl at the chokes because the barrels are very thin. As far as what you shoot, your limited because the sectional density of the 200 gr RN is low the bullet is short and walls are thin .030. I got some Hawks recently to try, but have not shot any yet. The deer I shot ran around 100yds before falling dead with no blood trail. The bullet opened up fast and did not exit. Next time I won't shoot for the shoulder as the bullet broke the shoulder bone and near side rib and left a hole about the size of a golf ball. Not a knock em down dead round for sure.
Dies are available from varoius places. The round is mild to shoot, about like a 35 Remington. I just got some old Peters 16 ga 2-9/16" rifled slugs to try in the right barrel, but have had no time to try them.