Actually they are overpriced and wear out depending on how hard the firing pin of a given gun batters them. Why not take a fired case, knock out the primer, and glue some hard rubber in the primer hole with super glue? Cut off a piece that is about the right size, glue it in, and then trim it off flush with the base of the case with a razor blade. Rubber erasers are too soft and crumbly. Like a chunk off an old tire tread would work. If you have a friend who is a reloader, have him make you a dummy round. No primer or powder. Just a FMJ bullet or something. Then fill in the primer pocket and perhaps drill some holes in the side of the case to make it visibly a dummy like the mil. does. Or also do something to color it like paint the bullet blue so that it is clearly the dummy snap cap round. I make snap caps that way for ammo that I have dies for. But like they said, gun shows sell individual snap caps, usually the red plastic and brass ones. But they will run you maybe a couple bucks each, maybe 3.