I was taught that dry-firing a C&B revolver would ruin (mash) the nipples. For a caplock rifle I put an old-fashioned faucet washer (yes, kitchen/bathroom faucet washer) over the nipple which absorbs the impact. Took a nipple to the hardware store to make sure the washer fit tight and bought a handfull. Have not tried this with a revolver, but it should work if you trim the sides of the washers so that they will fit in the nipple "channel".
If you try this let us know if it works.