Found this old bullet next to a Grist Mill Foundation in the woods of Vermont , just trying to get an idea of how old the site might be , thanks for looking !
Such calibers were used around North America for hunting (besides being used in the civil war), so no surprise in Vermont. Probably about the same time frame, though.
There really is no way to tell how old it is. You can buy a new bullet mold today and cast the exact bullet. Bury it in wet garden fertilizer and in one month it looks like it has been in the ground for 150 years.
My uncle deer hunted in the '70s with a replica 1863 Springfield rifle made in Italy and cast .58 cal. bullets for it. They were 3-ringers and dead-ringers for civil war bullets. Very effective; similar size, weight and power to a modern 3" 20 gauge deer slug.
I've found a couple of fired ones in Connecticut at very old park sites that were hunted a lot in the late 19th and early 20th century. Could be military surplus used for hunting after the Civil War or from a Civil War vet's rifle that he kept after the war for hunting... I guess not all Civil War period military items are found on the battlefields.