Looks like a 30/40 Krag used by US military. The .30-40 Krag (also called .30 U.S., or .30 Army) was a cartridge developed in the early 1890s to provide the U.S. armed forces with a smokeless powder cartridge suited for use with modern small-bore repeating rifles to be selected in the 1892 small arm trials. Since the cartridge it was replacing was the .45-70 Government, the round was considered small-bore at the time. The design selected was ultimately the Krag–Jørgensen, formally adopted as the M1892 Springfield. It was also used in M1893 and later Gatling guns. Hickok did a video on both the carbine and the standard issue. https://www.youtube.com/watch?v=Vnuy7zPfFAQ