The M102 Howitzer is a radially mounted 105 mm cannon. As it is able to swivel it is classified as a turret; however, the swivel range is only 15 degrees in each direction. The cannon takes a few seconds to reload between each shot, and does not support ripple firing if multiple cannon are unable to fire in the same direction as each other.
A typical use for this weapon would be on the side of a large aircraft, such as the AC-130 gunship. Presumably, the 20 rounds included with this weapon are considered stored inside whatever craft it is mounted on – although the 7.4 tons of mass they add are centered around the firing piece of the cannon.