I've been wondering about this one myself, looks like some sort of howitzer. The photo was definitely taken at the Seven Pines battlefield as the Twin houses can be seen in the background, indicating it was taken around Casey's Redoubt.
According to the
description at LoC, its a 32-pdr. field howitzer, however most of those I've seen used in the ACW had longer barrels and handles above the trunnions.