I have used 7625 and PB, but for over a year I have been using Solo 1000 at 17.5 gr, 7/8's oz and the 1100 pink wad. If you want it to be 2 1/2", cut the hull to that overall length, and prior to the first crimp stage put ina 16 ga overshot card. You will get a folded crimp, with an open center. You can read about the process and pressure velocity testing in Shooting Sportsman Vol XV Iss V

This ain't a dress rehearsal , Don't Let the Old Man IN