Now I will not call it stupid, yet.......... However this little guy has me wondering. Yes it is a complete cartridge. For those that think the .22 is a small round, it's a monster compared to this one. That is a CCI .22 LR Stinger for comparison next to it.
Some sort of pre-1900 parlor pistol round. Likely a BB Cap, maybe something associated with Flobert. Is it 4mm?