I dry fired my M500, like, maybe, a thousand times when I first got it. The firing pin retaining pin eventually broke in two, and the firing pin itself was chewwed up where it made contact with the retaining pin.
I don't dry fire the 500 too much anymore. The Mil-Spec 590 might be more robust.
TvDean,
My vote is for snap-caps.....did you know they make them with a wool swab in front. Oil it down, chamber it, & snap the trigger to release firing pin spring tension.
I have a trio of snap caps that I found in the local gun shop, they are made of a tough plastic and are great for practicing firing and shucking my 870. Load up the magazine and shuck and fire away, they bounce off the cement floor in my basement and suffer no damage. The caps are black plastic with a yellow primer.
Stay away from the clear plastic caps and brass caps if you plan to do this however, they break and dent easily