A deck gun is a type of naval artillery mounted on the deck of a submarine. Most submarine deck guns were open; however, a few larger submarines placed these guns in a turret. The main deck gun was a dual-purpose weapon used to sink merchant shipping or shell shore targets, or defend the submarine on the surface from enemy aircraft and warships. Typically a crew of three operated the gun, while others were tasked with supplying ammunition. A small locker box held a few 'ready-use' rounds. With a well-drilled, experienced crew, the rate of fire of a deck gun could be 15 to 18 aimed shots per minute.