The 3 grooves and hollow base sounds like a standard .58 "minie" ball fired from a rifle musket. Revolvers could be loaded with round balls, but ammunition issued during the war was generally skin, paper or tinfoil cartridges with pointed bullets. Revolver bullets ran .455-.460", so .452 would be about right once some of the lead corroded away.
I belileve regulations prohibit digging or using metal detectors on Park Service land, but there are still plenty of bullets on private property. And of course, if there is a shortage, one can always dig out the old mould, make a few hundred "orignal Civil War" bullets and bury them in the compost heap for a year or so.