Lots of good information here. I use them in all of my hammerless doubles--nitro or black powder. Spring loaded ones are superior. Others are crap. Those would include a plastic pellet in the primer pocket, a piece of pencil eraser, or even a drop from a hot glue gun. One thing NEVER leavet hem in a rifle when you go to the airport. TSA won't know what they are and the place will go into a panic or lockdown. While I never had the caps in a rifle, years ago I had a set in the gun case with other hunt gear (sling, ash bottle, loops, etc.) and it was almost impossible to tell TSA they were not ammunition.
Cal
Cal