Tom,
I would assume it would be a Tennessee style bullet but there isn't a cavity. Not really sure but there are several guns that were used at Antietam that could have fired it. I still have buckets of the stuff to pick up and many more jars, boxes, cans, etc. to go through that are full of bullets, plates, etc.
Mike,
One interesting thing is that you can see where the CS troops were later by the batches of Macon (flat base) bullets and .54 Kerrs. I'll see what I can come up with while going through the piles. I have several days of washing bullets ahead of me as most of the stuff isn't even cleaned.