[Phantasm]

Just as an improved beam weapon can be said to have multiple barrels to double the RoF, an improved gun could possibly be a gatling with multiple rotating barrels - proven in the real world to have a greater rate of fire over single-barrel guns - or be two or three guns of the same caliber side by side and treated as a single "gun" as far as the stats go.

For a real world example, consider this: Are the gun turrets on the WWII Iowa-class battleships medium batteries, or improved major batteries with three barrels? As you have to aim all three barrels at the same target and commonly fire them at the same time, I'd go with the improved major battery on each turret.