I've been down the same road with trying to find a viable substitute for factory made percussion caps. The bad thing about using the plastic ring caps and the Forster Tap-O-Cap is that the ignition compound used is very corrosive (or perhaps erosive is the proper term(?), help me out you chemical engineers.)
Anyway, remember as a kid how those nice shiney nickel plated metal cap guns looked like they were dredged out of a swamp after just a little bit of use?!?! If the stuff in the caps is Potassium Chlorate (as I've been told), you better do a very quick and thorough scrubbing of your gun(s) as it will ruin things in less than 24 hours.
FWIW, I never got consistent ignition with the plastic ring caps, in spite of the fact that they fired every time. I guess there is just not enough priming compound to insure positive ignition of black powder. BTW, don't even bother trying to set off Pyrodex loads with them. (I had even less success with that!)
Regards,
WV_Hillbilly
PS Keep looking... one of these days, some one is going to come up with substitute caps. I hope I am still around to see it.