My caps make little to no smoke. I try to use only RWS brand on my rifles and shotgun. The smoke you're seeing upon ignition is most likely powder smoke from the ignited charge that leaks back past the cap. They don't usually fit tight enough to seal off the pressure coming back through the nipple. Often the caps will fracture upon ignition and would leak even more.
Before loading my clean gun I snap a cap on an empty chamber with the muzzle near a blade of grass, etc., to be assured the nipple passageway is clear into the breech. The blade of grass will move a tiny bit from the force of the cap's charge exiting the muzzle, and you will hear a hollow "thunk" from the barrel. I have never noticed any smoke from RWS caps when doing so. 1800s caps may not have been smokeless.
