I would do as Brian suggests. I've also heard of guys knocking out the spent primer and inserting an eraser off of a new pencil to use for shock absorption. You could also buy some inexpensive snap caps, no need to pay a lot of money for them. Just about all the snap caps now a days are painted a bright color, the reason being so as not to confuse with a live shell.
If you do use a fired hull, I suggest you paint the base for easy identification. You don't want to think you have a snap cap in the chamber when in fact you have a live shell
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