The artillery piece is a bronze Napoleon, probably a 12- pounder, the most common variety used in the war. The Napoleon is a smooth bore gun; it's greenish tinge is a giveaway of its' copper (in bronze) composition. Napoleons are frequent pieces seen on Civil War battlefields- this one is a beauty.
Was at Antietam a number of years ago, taking my son who was doing a school report on the battle. It is wonderfully maintained, interspersed with sections of private property, also well maintained. After reading for years of this battle, Dunker Church, Miller's Cornfield, Hagerstown Pike, Burnside's Bridge, Bloody Lane, the horrific casualties - what awed and amazed me was the compactness of the field, everything is close by and pretty much visible from where you are. Unlike Gettysburg with its broader spaces, everything at Antietam happened pretty much right underfoot. A beautiful field now left us of what could only have been ugly ugly horrible days.
Haven't been to Shiloh, but I understand it is like that there as well.