I cant imagine the case would fail unless there is something drasticall wrong with you chamber or breech block I dont know if I would use a double wad. Juist seat the balls on top of each other. I have never tried double ball load at all, not even in a M-Loader. You are pushing 1000 grains there, might start real low and work up a load. Your accuracy might discourage much testing. I once tried a buck and ball load in a 12 gauge flintlock, the gun weighed under 8 pounds. A 715 ball and six buck, about 850 grains and 70 grains of FF. Didn't want to shoot another!!! A standard 12 gauge paradox slug is 780 grains,,,,Ed