You have to think a bit differently in terms of Medium guns. All or most light guns are very singular. By that i mean they have one functionality.
Merc shoots a sniper bullet. Disables components and does hull armor dmg. As simple as that.
Gattling shoots quick and consucetivealaely runs armor down. close range As simple as that.
Flamethrower flames.
Mortar blowy uppy stuff.
etc.
etc.
Now go back to medium guns.
Lumberjack, fires a long ranging projectile that blows up baloons in a great arc. Aims super high and low. Very specific to its left and right. Has an ironsight that helps the gunner determining the shot.
Twin carronade, blows up baloon in close range like a shotgun. Demolishes parts if aimed at that instead. Requiers very close range.
Hwacha, fires a barrage of rockets that disable and explosive damage. Used for both killing and or disabling. Long or short.
Heavy flak, hits hard to almost full kill when armor is down. Shots very slow and needs to be in atleast medium range. Bullets slow etc etc.
So when you say. A medium gattling gun that is as simple as 2-4 power of gattling guns then youve just defeated the light version.
My idea is a cannon: Shoots a big hitbox cannon ball that deals piercing and impact damage and hits the ship like it got rammed (like hitting a mine). One shot, long reload. No arming time.
My second idea is Bullet hunter: Shoots a short range very very close range aray of shotgun shots that eliminate any oncoming projectile. Does disabling damage only. Is a completely new gunning game for the gunner to fire the gun at projectiles from the enemy to protect their own ship. Fair Reload. Does not work versus non projectile type (Gattling, flamer, flair)
Oh my third idea is Air blast: I giant toy air concentrator... just kidding. But a gun that blasts for 0 dmg but flips other ships like they where hit by a moonshining galleon. One shot, very long reload. Somewhat invisible shot that is slow. Goes from short to very long range.
Fourth!!! Drone launcher: The gunner launches a small flying drone plane that he steers. Shoots gattling shots at 50% rate of the gattling firing speed. And right click disabling artemis rocket that does only disabling and 50% of that damage from the artemis disabling side. The ammo on the plane is limited so no reloading on the plane itself, but it can crash onto a ship for explosive, 150. A new drone plane reloads on the gun that launches it slowly when one expires.
The fourth is most interresting. I mean... think about it on a galleon... ooooooooo