I was reading the thread on Black Powder Reloading for Double Gun and started to think about the gas seal. I use 3 drams FFg a 1/2" lubed fibre wad, a plastic shot cup with the compression component cut off, 1 1/8oz shot, an over shot card and roll crimp. Hull is Cheddite 3" cut to 2.5". I've been using this recipe for 20 yrs in all my old doubles and have not experienced any problems. I have never sent them in for pressure testing nor have I tested the fps. Of course some of my guns pattern better than others. But I am wondering if the plastic shot cup provides any gas sealing characteristics? I am thinking upon firing, the lubed wad jams around the edges of the shot cup and any pressure on the base of the shot cup would flare it against the barrel walls aiding in preventing gas leaks.
