In this case, a stash of ancient RWS .22 rimfire BB caps. Tucked away amidst other oddities and conundrums, a few tins of these came to light as I was unpacking some ammunition brought home from storage.
Made by RWS of Nuremberg Germany, this ammunition might date from the 1920's. It's difficult to tell since it was repackaged for sale by J.L Galef and Sons of New York City.
Driven solely by the priming compound, it has a 19 grain round lead pellet driven at a nominal 750 FPS (an optimistic estimate in my opinion). Used for a gallery round, indoor target shooting, and garden pest control, the BB cap has power about equal to an older .22 air rifle.
Here the RWS BB cap is lined up in comparison with a .22 short and a .22 long rifle. It won't feed through most .22 firearms and must be single fed. For someone with large paws (like me) it's a fiddly round to load.
Look for a future velocity and accuracy comparison, when I get the chronograph unpacked (g).