For as long as we have had silicone seal I have made snap caps. Cut a shell to whatever length you want, I make them 1.5" long so they don't "feel" like a shell. Punch out the primer and put some silicone in the primer hole. (the better grade of silicone, re. automotive application stuff will last longer) Let it cure for a few days and trim it off flush with the shell head with a razor blade. It will last a long time and cushion the firing pin well.
Couple of guys at the club seeing mine made some from some Federal hulls that are pink. Think they are breast cancer funding promotion shells. Regards, John Hancock