So here's why Gunners suck;
1. Engineers pick three below, gunners pick one
a) Buff Hammers provide a 30% bonus to damage.
b) Spanners rebuild the fastest.
c) Rubber Malletts can keep your gun at 100% health, critically important now that gun health relates to effective output.
d) Chem Spray almost completely removes the need for heatsink.
2. Gunners get three ammo types.
a) Most guns only ever need one ammo type regardless of situation.
b) Even if responsible for two guns, most guns share a common ammo type synergy.
3. Engineers are flexible, when they aren't shooting they can repair engines, buff the hull, ect.
4. Gunners are fundamentally dependent on engineers, and cannot operate a gun without adult supervision. See Point 2.
Here are scenarios you can get away with a Gunner.
-Manticore or Carronade Goldfish. The Manticore appreciates heavy clip until point blank, when burst is valuable. The Carronade would benefit from heavy/lesmok at range and then charged, burst, or even lochnager closer.
Counterpoint: This same task can be achieved with two engineers each bringing a different ammo type, and a small amount of coordination. The benefit of this is the person on the gun doesn't have to be babysitted.
-Lower Deck Galleon with different guns on the left side than the right. Two different medium guns often requires preloading with two different ammo types.
Counterpoint: But usually there's a common ammo both guns like. The other person downstairs with you surely wouldn't appreciate having to keep rebuilding/repairing/extinguishing (pick two) the gunner's gun.
So why do people still play Gunner?
-Because if you play Gunner in a casual game, people will give you a gun. I do it too, because off a gun a Gunner is completely useless. But that doesn't mean they're more useful on a gun than an engineer.
-That's it. The above reason. That's it. That's the only good reason to play a Gunner now.
How should Gunner be fixed?
This isn't an RPG (outside of Burning Skies), we don't need static value bonuses some classes get over others. Here's my suggestions.
-Change the Gunner from 3 Ammo types and 1 Repair tool to 2 Ammo types and 2 Repair tools.
-Give the Gunner access to unique ammo types. Can you imagine how different the game would be if only Gunners got Lesmok or Heavy Clip?