Personally, I don't think the oxidation is enough to be Civil War period. Every bullet I've ever seen from that timeframe is typically white. I could be totally wrong on this, though. Definitely has the miniball type characteristics, though, so I can see the question as valid. Hopefully, someone else will chime in.
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Unless it was in an area that is generally wet or underwater im with skippy. Of course if it werent fired or as smushed you would be able to tell by the shape of the head of the bullet.