The ARS-G04 Gun belongs to the military AIRS set. It fires a close-range anti-air/anti-ground shot on the ground, and several rounds from the air. Overall, this gun is not very useful.
Use the ground shot to counter advancers and use it on enemies who like to Low Air Dash since hitting the bottom round from the top will knock the foe into the top round and vice versa. Use the aerial shot to stop aerial advancers when you are airborne or use as an attack with your Lightning Sky's sky-dash on slow moving ground robos.