It is a British .577-caliber Snider-Enfield bullet, used from 1866 into the late-1800s. Many of these bullets have been dug in Canada, and a (very) few in the US, which has caused some diggers to think they are a "4-ring/groove Minie bullet" from the American civil war, but they are definitely from no earlier than 1866. https://en.wikipedia.org/wiki/Snider–Enfield