The advice to disregard chamber length and use 2 7/8" hulls only works if you do not have square corner chambers. Make sure you have tapered forcing cones before you try to run longer than chamber length shells in there. I see nothing wrong with using 12 gauge target load data in your shells, using powders on the fast side to maintain adequate pressure in the big hole. Use something faster than Herco with 1 1/8 ounce loads to insure good ignition. Green Dot would be my choice in a very old gun. If your barrels are relatively thick, I would use Red Dot.
