It is a iron ball a little larger than a golf ball. I assuming that they put several of these into the cannon and fire them for anti-personnel. Kind of like cannon buckshot.
The brass item is off of a saddle. I have several of them, and seen several of them on complete saddles. Howard Crouch's book has them labeled as a "saddle guard", if I remember correctly.
Cool finds. Several people are right on. the brass piece is to a saddle and was held onto the wooden tree by 2 small brass screws. the bullets is a sharps carbine and the iron ball is a canister shot from a cannon. I think they fired like 16 at a time or so unless it was a double charge then it would be 32. But like you said it would be like a shot gun. I'd hate to have those thing flying at me.