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.   

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 32 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 9 propellant per barrel, the cannon has a 270 block range).  

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

Backstory:  
After the success of MCST, the Slokovian Worker's Union sought after the power of owning one of the strongest cannons ever invented: the Goliath Main Battery. Using the schematics for the MCST as a base, the engineers designing the cannon increased its size by 60 cubic meters and mounted eight different projectile sources in the dual barrel turrets of the MCST. What they invented was a nightmarish gun that fired an astounding 32 TNT per shot at incredible ranges. Her size meant that she could be mounted on a variety of medium sized or large sized warships, facing the nations around them with the terrifying prospect of facing a battleship that could both severely outmanuever and outgun all other heavy battleships with its firepower. Due to the sheer resource consumption of the cannon, little of them were produced, but when mounted on a warship, they would truly be a force to be reckoned with.