there is alot more to this than what meets the eye.. make sure if you shoot one barrel you reseat the other wad column and over shot card with ramrod.. repeated firings can loosen the load and caouse a dangerous situation.. most shooters will pull off the cap before sticking a ramrod down the bore, both capss in some cases if fully loaded, its not worth lossing a hand over a couple seconds.. what i did when i was shooting heavey 10 gauge loads was to use 2 10 guage over shot cards ( that would be 12 guage in your case) with the punched edge up.. there is a tiny rise in one side of the card when its puched out, this side would go up as you load it into the bore and two of these will hold quite well if the over shot cards are made right.. if for some reason this doesnt hold you will need to reorder 12 guage cards from another source.. it happens but its rare.. i had trouble getting proper sized nitro cards for my 10 and had to make a punch, and punch out picture frame matting the exact size i wanted but i was competeing at the time and was very particular.. three picture frame matting over powder, fiber wad, shot and picture frame matting card over shot would probably work fine, but i used the purchased white over shot cards as they were correct size for me in that case.. good luck dave.