Found this a couple days ago about 4-5 inches down in the mud. It looks like whoever shot it missed and the bullet hit a rock maybe? Not sure how old it is but could it possibly be from the Civil War era or even older?
Appears to be a pretty typical cast lead bullet, which means it could be anywhere from quite old to fairly recent. The damaged area is probably where it contacted the ground when it came to earth which, as you mention, could have been a rock.
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