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.   

Dual Shot: This specific TNT cannon fires 2 TNT of projectiles per round. It uses a comparator locking system to dispense the TNT projectiles into the blasting chamber on a delay.   

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 10 propellant, the cannon has a 310 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 advancements of the many scattershot cannon designed by their engineers, the Slokovian Worker's Union realized their use of scattershots on nearly all of their warships would be disastrous if they were on a tight budget. They set to work building a more resource friendly cannon than all of which they built before. They decided to incorporate the dual shot technology into their cannons, however they noticed one flaw in the dual shot cannons employed in the War of Acquisition: Their range could not compare to the scattershots that they used. So they decided to install two more propellant dispensers in their standard firing chamber, giving the cannon ten propellant instead of eight. This gave the dual shot a range of over 300 meters, which was more range than almost any other cannon they utilized. These turrets were mass produced and stored in warehouses throughout the union, waiting to be mounted in case their ammunition supplies were depleted to dangerously low levels.