Ive came up with an answer that will protect your gun and allow you to practice squeezing the trigger. I take the brass from the gun I plan on practicing with. Push out the old primer and replace it with a cross section from an "O" ring that fits snug where the primer goes (bigger primers Ive cut pieces from garden hose gaskets). You can put the empty brass in, snap the trigger how ever many times you want, and you will not damage your gun.

I forgot to mention, you usually need to resize the brass, I even put a cheap bullet in them form my autos.