I think there's no need to worry that you found a modern Blackpowder deerhunter's Repro minie-ball. The modern ones are nose-cast, and yours isn't nose-cast.
Regarding the lack of oxidation/patina... civil war bullets found in "wet ground" (and underwater) often show no oxidation, because the water prevents oxygen from reaching the lead. During the past 30-something years, I've found a good number like yours in creeks and swamps.