This cannon requires the Spigot/Bukkit server plugin Movecraft to make the turret turn. The redstone will work without any mods or commands.  

Features:  

Adjustable Arc: The arc can be set on the cannon from the repater next to the trigger button. The cannon has three different arcs to choose from.  Note that the belly turret requires a hole cut out of the airship's body for the turret to fire at its highest arc. 

Compact Size: This turret has incredibly small size due to the redstone that makes the entire cannon work. Using advancements from some of the best cannon designers in minecraft, this cannon is easy to place as it consumes little volume around it.  

Scattershot: This specific TNT cannon fires 8 TNT of projectiles per round. It uses a comparator clock to rapidly dispense the TNT projectiles into the blasting chamber. 

Compacting Water Stream: This feature only works with minecraft PC versions 1.8 or higher. Using water to push all the propellant to less than 1 block from the projectiles, the cannon has tremendous range for the amount of TNT it uses to fire(Using 8 propellant, the cannon has a 240 block range). 

Turning Capability: Using the Movecraft Plugin, the cannon can turn between four different orientations. This is the only feature that will not work in minecraft vanilla versions.  

Backstory:
After the two successes of Henry Geolovich for the Slokovian Worker's Union, he was once again called upon to build a turret with the same mechanics,. except this time the cannon was to be mounted under the airship instead of on top of it. Henry set to work using the same mechanics that his previous design used, only moving around the wiring so the turret could fit under the airship with minimal difficulty. He succeeded in his design, and the turret was pushed forth into the industry of airship construction.