Heck- I figured it fired a large CLGP type of shell. The railgun tech gets it moving pretty fast, but the projectile is either self-guided or externally guided to it's target. I carries it's own rocket motor for the final acceleration to the target and for course corrections, because the launch of the round from the gun itself is not as fast as it could be launched from another platform, such as a ship or a land-based gun of the same size. Also- as there is no need for a reciprocating breech, so maintenance is minimized. Finally- if the tech for a good, solid working railgun is there, some form of recoil compensation should be possible as well.
When you mentioned Cannon Launched Guided Projectile I was thinking of the Copperhead which looks nothing like a shell and more like a missile. It is shown briefly in the recent alien invasion movie Battle: Los Angeles.
I was also thinking of an advanced form of Half Life 2's zero-point energy manipulation (Gravity Gun). It is also believed to be the kind of tech that UFOs use to make on the dime 90 degree turns at 20 times the speed of sound without breaking up. The tech is applied here to minimize recoil that would otherwise damage the craft.
That was my first thought when drawing out the gun. I don't have a suitable fictional name for it yet, but the tech behind the gun is in the same field as railgun/metal storm technology. The gun still has recoil but its minimized greatly while allowing shells to fly extremely fast.
The great energy drawn by the gun comes from the helicopter's nuclear-type engine (through those cables with the colorful plugs).
The engine also powers the helicopter's huge rotors and allows it to patrol the seas without landing for a few years. Its primary role is to go up against a well armed and powerful enemy navy fleet with advanced laser defenses.
Until the 'Hammerhead' was built such a fleet was impossible to attack with existing aircraft.