Are you in an area where there could have been a Civil War fight? Your find is just plain dripping with age. The lead in the wood keeps that part of the log or limb or fence post from rotting away, preserving just that small bit by the bullet, and it can be quite old. Say a bullet hit a tree 150 years ago, and the tree died 75 or 100 years ago, then the rotting process returned the tree to forest litter, but the lead preserved that small chunk. You have a unique find, and I wouldn't do a thing to it but display it where people can't handle or pick at it.