We're having this discussion over and over and over and over and over again....
pandatopia pretty sumed this up, the idea is everyone can do everything equally well but classes are limited by possibilities - therefore gunner has 3 ammo slots and 1 engineer slot - but essentially they use the same tools (gunner shooting the same gun with the same ammo is no different than engineer or even pilot shooting the same gun with the same ammo). And as far as I know Devs will do everything for things to stay this way. There is only one thing that bothers me:
Who cares if 2 gunners is not good - why do you have to be a gunner?
Well, game gives you choice between three roles - implying that those roles are more or less equal, pretty equally needed on the battlefield. But then it turnes out that pretty much one of the role is inferior in the game and you can only choose between other two. If most competetive (and experienced-pub) scene is sure that crew of the ship consist of minimum 2 engineers - this is simply bad game design. Similar is with "extinguisher or chem" question - there is no real choice, you don't see high-lvl engineers running with extinguisher because it's purely inferior to chem. Game doesn't give us real choice regarding this matter.
Being able to carry three engineering tools is already a passive buff that engineers have making thm better at repairing than gunners. Being able to both put out fires AND repair AND potentially buff is a massive benefit over a gunner.
Because engineer is for repairing and gunner is for shooting. And maybe two gunners will never be "viable" choice because the risk of taking a second gunner simply isn't enough to pay-off the safety of second engineer. This may be fundamental game feature/error which can't be "fixed", but IF it can - the simpliest to implement and best solution is to make such ammo types that are worth the risk.
I do also like the faster reload speed idea a lot. Or the possibility of being able to select an ammo type and walk away and it still loads that ammo type. Having different ammo types is sort of helpful but it can be crippling to have to stay at your gun to load it.
On one hand - yes, this is an interesting idea that can increase the use of gunners somehow. On the other hand if one gunner can pre-load all of the guns in one run (just like engineer chemsprays) - second gunner seems unnecessary again. Also, technically you don't have to stay on your gun all the time it reloads, only like second or two before it ends and it's used many times in the game, so basically it's just cooldown thing, just as engineer has to look out for cooldowns.
If anything, gunner needs another tool type. A tool type that activates when selected on a gun so you can combine it with the ammo loaded.
Devs always say - gunners don't have tools. Gunners have ammo.