Fyrffytr1 is correct. More specifically... your bullet-casing has the indented-lettering version of the United States Cartridge Company's "US" headstamp, which dates your casing from about 1909 to 1936, when that company's production ended.
Comparing your casing's size with the ruler, the tube which held the bullet seems to be approximately 13/32-inch wide. Along with the casing's extreme shortness, I believe it is a ".41 Short" made for use in a Derringer pistol.
Edit-note: I see BosnMate and I were typing our replies at the same time. Mine took a bit longer because I had to research the indented-US headstamp's dating.