/r/whatisthisthing would be a useful place to ask this as well.

If it's iron, it's unlikely to be a shot. Those were typically lead.

http://s220.photobucket.com/user/dcb1265/media/2-13-6b-1.jpg.html

http://www.treasurenet.com/forums/attachment.php?attachmentid=637423&d=1337428911

http://lh4.ggpht.com/lavotini/SC4Gk2p2FGI/AAAAAAAACak/Xw2BTJthqdI/064_thumb%5B1%5D.jpg?imgmax=800

http://i53.tinypic.com/20u3cl4.jpg

Sure looks like it!

Grape shot? Fired from a cannon possibly?