You don't need to buy a snap cap. Since there is discussion on using spent shells, I thought I would go a step further.
On simplified way of making your own snap caps is to take a spent round, use a nail (or decapping die if you have one) to remove the primer. Take an electric drill to enlarge the primer hole. Cut up an old "pink pearl" eraser and stuff a piece of the eraser into the enlarged hole (if you have extra time glue it in) If you don't have some suitable scrap rubber material, some of the poly-plastics will work, although some will permently deform after a while and then need to be replaced.
Then you have your own home made snap cap.
If you want you can also put a bullet into the spent case so that it will feed from the magazine. (if you don't have a reloading die & press, you can use a regular bench vise on a short semi-auto cartridge). You might want to paint the snap cap for easy recognition.
