Intended just as some friendly corrections.
First... actually, the great majority of musketballs used in the civil war were .69-caliber, not .58-caliber.
Second... the bullet cannot be a .45-70 bullet, because the photos show its diameter is between 10 and 11 millimeters (approximately .40 to .44-inch). But .45-70 bullets were made for use in a .45"-bore Breechloader rifle, so they measure approximately .47-inch in diameter.
Also, all of the .45-70 bullets I've ever seen have only three body-grooves, not four, as the photos show on this bullet.
Trying to help ID it correctly, I hunted through all the .40 and .41-caliber bullets at the Cartridge Collector website. But none of those calibers there match up with this bullet's peculiar shape of nose. It isn't flat-tipped, and it isn't "round-tipped" either. Best description I can think of is a "
blunt-ish rounded" nose.
There seems to be no exact .40 or .41-caliber match for it at Cartridge Collector.