Plastic for preference but the paper rolls will do in a pinch....pinch two off and a bit of sticky tape to hold them on the nipple, works well on military nipples.
Don't think that 'the powers' are aware that they exist so I presume that there is no penalty for using them...not yet anyway
The paper rolls are really Maynard's Patent Pellet Primers.
These caps are highly corrosive so only used when out of proper caps.
I have one gun that was designed to use the paper caps. It is a simple tube gun and has a touch hole underneath the barrel and originally had a wooden hammer and trigger with elastic band spring, but Customs took the woodwork when it was imported from Rhodesia, or so I believe.
Eoin.