Except I don't want to play this way, it's simply what's currently effective. I don't want it to be effective, and that this thread exists means a lot of people are unhappy that three engineers are so effective, it's something that should be a novelty setup like three gunners or a flame pyramidion. And nowhere did I say that Shinkurex was useless, just that in my (and many other competitive players') experience someone that can shoot well is more useful to the captain as a gungineer than as a gunner. The role doesn't change the players' skill or actual job on the ship at all. Someone that's good with ammo switching is obviously going to be more useful a gunner than one that isn't, but there isn't enough emphasis on that skill in the game right now and that's the issue.
I'm not arguing that those who currently use gunners shouldn't, but that just because those people CAN use gunners doesn't mean there isn't a need to tweak the mechanics to make gunners more appealing to more than just half of the competitive community. I think a lot of the problem has to do with the current penalties to ammo switching, as even before this thread was made gunners apparently were more in demand because of heatsink (a defensive, chemspray-like ammo) than actually being able to switch to the most effective ammunition to fit the situation.
Ammo right now is tailored to specific guns and there's an expectation that each player should only be on one gun (two on a Galleon), the result is that more often than not whoever's shooting isn't making use of all three ammo slots. If the mechanics could be rebalanced a bit to encourage more ammo switching then the meta would probably start to favor the gunner over the gungineer.
Just to give an example, let's say a Pyramidion has two engineers and a gunner. Both engineers are on repair because the ship's taking heavy fire, the front guns are a gatling and flak. This SHOULD be the perfect situation for a gunner, strip off armor with the gatling and run over to the flak to smack their hull to distract the enemy crew off of your ship; however, the current penalty for leaving your gun during reload makes it so that the gunner can't really make use of his multiple ammo types and stay equipped for both guns, he might as well have just been a gungineer with heavy clip in that situation. You CAN do the gun-swapping trick on a Galleon with double Hwachas and still make use of the Gunner's unique traits, but right now that's the only setup where that's possible and that's a damn shame.
Apparently the way reloading works right now is due to some UI limitation rather than any clear vision of how the Gunner should work, though I don't see why defaulting to vanillas is going to be any clearer than defaulting to what was last left in it.