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 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 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 many successes of the engineers in the Slokovian Worker's Union in compact cannon technology, the designers and engineers of the Union set their sights a dual barrel scattershot cannon. Using the CST as the base model for this cannon, the designers implemented the advanced comparator locking technology and propellant blasting gate to create a small, powerful, dual barrel scattershot cannon. The result was the MCST built by the engineers of the Union. Their design was a full 90 cubic meters smaller than the original cannon and had significantly greater range, not to mention it consumed two less TNT per shot than the original model. The cannon soon became the standard dual barrel turret for all the designs in the Union that used this kind of scattershot technology.