The cartridge on the left is a .41 Short/Derringer, for comparrison.
The right-side cartridge is supposedly a rimfire, with a bullet measuring .299 in diameter, a case measureing .758 in length and .344 in diameter. The headstamp is a ‘U’.
I do not have the cartridge nor have I held it, all I have is what you have, the picture and the measurements. Ron has suggested it might be European or perhaps even for a toy rifle (dummy). Any help would be appreciated.