I think Timekiller is correct. Here is more-detailed info for you:
Your bullet appears to be the 500-grain (weight) version of bullet made for use in the US .45-70 Government Rifle, which was the primary US Army infantryman's longarm from 1873 to 1893. The 500-grain bullet for it dates from 1879 onward.
Although the US Army replaced that rifle with a different kind in 1893, the .45-70 continued in Army service hrough the Spanish-American War (1898). Afterward, many thousands of the .45-70 rifle were sold to the public as "war surplus" ...so, .45-70 ammunition continued to be manufactured and sold in large quantities. In fact, newly-made .45-70 bullets can still be bought today. However, the 20th-century version has a copper "jacket" on the lead bullet. The version you found dates from 1879 to the very-early 1900s.
If you're interested, you can learn more about the .45-70 Government Rifle and its ammunition here:
Gunczar.com .45-70 Government History and Information
Here is a photo showing what your bullet looked like in its cartridge. It is the third one from left in the photo.