Wont harm them initially...but the spent primer of a fired cartridge will not be much use after two or three hits strikes as it will have dented and deformed to the point of being useless in the capacity of a snap cap. Snap caps have a spring or a elastic material inside of it which ensures the "faux-primer" returns to the proper depth/position to protect the firing pin.
Natchez, Brownells...even Amazon sells 44-40 snap caps.
NRA Life Member (2003)
I think that one of the notions common to the anti-gunner is the idea that being a victim is 'noble'; as if it is better to be noble in your suffering than disruptive in your own defense.