Interesting concept I've been pondering for a while here. I love to hunt with my Marlin 45-70 but I love to shoot grouse as well, although Im a big shotgun fan as far as they go. Speer makes a .451 cal shot cup for colt .45 revolvers so you can handload snake popper rounds, (more intended for the states where mounted riders carry sidearms for just such situations). I've found each cup will hold 167 grains of number 7.5 shot. I came up with a charge and primer to go with it and loaded a bunch up. The diameter was a little tricky to crimp due to the .007 thou size difference between colt .45 and 45-70 but I got round that. I intend to pattern sometime soon and will let you know how it turned out. If this works the shells should be a great grouse popper load.