Any 2 3/4 ammunition you desire or is proper for the gun. When Parker and others chambered 2 5/8 they intended the gun to use longer 2 3/4 hulls. I think the reasoning was the hull would partially open into the forcing cone and provide a better gas seal as the fiber wad moved from hull to bore.
Parker hang tags on new guns said, Chambers 2 5/8, next line said, use 2 3/4 shells.
Pretty clear.
Joe, at what point in time did Parker make the statement about using 2 3/4" shells in guns with 2 5/8" chambers? The reason I ask is that SAAMI pressure standards aren't the same now as they were when the organization was established back in 1926. I don't know for sure and I don't want to assume, but my guess would be that at some point, as the industry standard (especially in 12ga, which is where the change occurred first) changed to 2 3/4", Parker began boring 12ga chambers at 2 3/4" rather than 2 5/8". Thanks.
