This topic has come up several times on GBO over the years, once or twice i think I even posted the history of one company from its founder through three owners, so a search would net you a bunch of information.
I bought a bunch of them 20+ years ago for lots of callibers , for CF to RM, CF to CF and both CF & RM to pellets (they used standard primers for propulsion). Some were made specifically for TC's that are easier and faster to use as they don't have the firing pin plunger that has to be removed to reload them, but all of them are slow unless you buy a bunch for each firearm, and most of them are not very accurate. Best of the best out of all of the CF to RM conversion for me was a 22 Hornet to 22LR modified TC type case in my 14" 22 Hornet barrel. While it shot low naturally from the scopes setting for the CF round, it did shoot very acceptable groups out to 50 yards. A 222 Rem to 22LR did nearly as well. Some shot OK in bolt rifles as well, good enough for serious plinking and some for varmints. The CF to CF conversion did much better in my TC's though.
