I am 100%-certin that the bullet is not a .45-70 or .45-55 (etc), because it does not match the profile and length of those bullets. Its profile, length, and grooves do match up with an 1866-to-1870s-era .50-45 or .50-70 Government Rifle bullet. That ID would seem to be confirmed by the description from its finder (Unclemac) that it is a half inch in diameter. To see one (which is identified as a .45-50 bullet by a US National Park Service archeologist), go to the following webpage's .pdf document showing bullets & cartridges recovered from the 1876 Custer's Last Stand battlefield. Scroll down to page 46 to see the photos of the .50-45 bullet (and some .45-70 bullets for comparison).
http://www.nps.gov/mwac/publications/pdf/tech94b.pdf