Here's the issue: buffs are permanent while stamina is temporary. This would benefit buff engi much more than gunner. Their guns would turn faster, have better arcs, and higher dps from the reload bonus. In order for gunner to compete they'd need a sizable damage boost, but this isn't balanced because gunner stamina is temporary
Take the gat for example. It has a long empty clip time so you'd need to use a lot of stamina. The hwatcha however has a short empty time which means you'd only use it for a few seconds. Or the mine launcher where you'd use stamina for just a single instant
The current system works because turn speed, arcs, and reload are good temporary utility bonuses while damage is a simple permanent bonus. The gunner is for utility and gunner stamina reflects that. If +20% dmg is too much then the answer is to adjust it - not change everything else
You are making no sense here, and are only focusing on a single aspect of the suggested change. You are also ignoring how stamina actually recharges. Guns turning faster, having better arcs, and reloading faster would buff an engineer more? As opposed to the current buff that makes DPS gunners obsolete? And at the same time you are saying the change to stamina would make gunners overpowered on just the hwacha because reasons.
If you are so afraid of Burst ammo, then add some jitter to it. Problem solved (falloff is a better solution). But, but then engineers would have a harder time shooting it!!! They are engineers. They are supposed to have a harder time shooting guns than actual gunners. What a concept.
As for longer firing guns like Gatling, breaking things recharges stamina. Gatling is good at breaking things. Gatling with more damage and tighter recoil breaks more things. Stamina recharges faster. Not fast enough? Change it. They just have arbitrary numbers as it is. Also of note for the bad example you gave, Stamina currently lasts about the same time as unloading a Greased ammo clip. When shooting at hull and components with buffed Greased, things break fast enough to give you a full clip of stamina firing. Other guns could have a better argument against it, but still not a good one. The same argument can be made for the arc changing. It benefits fast shooting guns over longer clip guns. It benefits fast moving guns with good arcs over slow moving guns with narrow arcs. What is your point? It just seems like you don't like the idea of temporary damage increase that removes the buffgunner class from the game. No one is going to take a buffgineer over a stamina DPS gunner because the buffgineer has a little better arc ability. People will take them to supplement the gunner in the same way they supplement the pilot or another engineer. They will also take them for creative builds using the arc changes, which is far more useful than DPS increase.
Gunner stamina, as it is, is a melee thing. You can use it for arcs or use it for reload. Not both. The reload bonus is minuscule on long range guns. The arc changing is practically useless at range. These changes would make it all around better.
For the record, I would make the change to the arcs smaller than gunner stamina and leave the movement boost about the same. Not sure about reload. I would also increase the buff duration since it is no longer an active player tool, meaning it will be used more by players other than the one doing the buffing (hint, the gunner).
