Lucas, thanks for measuring the bullet's diameter. That information enabled me to identify your bullet with certainty. It is a US .50-70 Model-1871 "Government" Rifle bullet. (That rifle is sometimes called a Model-1871 Springfield Rifle). Here's a photo showing an original .50-70 bulletmold from 1880. Also, here's a diagram of the .50-70 Model-1871 "Government" Rifle, which shows your bullet in its metallic cartridge-casing.
As Gunsil indicated, when US Army rifles were superceded by a newer model, many thousands of the obsolete ones were sold to the public for big-game hunting. Then as now, a good number of hunters reload their used cartridge-casings. That is why Winchester made the .50-70 bulletmold in 1880.
Almost forgot to mention... your bullet has ben fired, even though it shows no impact damage. Your photos show rifling-marks on your bullet's sides.
I see you are a brand-new TreasureNet member... so, welcome to T-Net's "What Is It?" forum, the best place on the internet to get unknown objects CORRECTLY identified.