It is a copper-jacketed lead core bullet, which means it doesn't date any earlier than approximately 1895. The fact that it is a "hollow-point" copper-jacketed bullet strongly indicates it is from no earlier than the World War One era.
It is quite long-bodied, and has three flat-bottomed body grooves, and it does appear to be approximately .45-caliber, so it probably is for the US .45-70 "Government" rifle. Although long obsolete, some .45-70 rifles did get used for training "raw" (newly-recruited/drafted) US National Guard troops in the World War One era.