Normal Rounds.
This.
So much this.
Why did noone say that before? o.O
Why did I not say that before, thinking about it today?
I was thinking about it earlier today (all the hive minds), and how would the execution look like, for we'll end up with a problem of not having a basic ammo at the very start of the game and reload mechanics would have to change a little bit if there's no basic ammo for a gun to switch to if the user isn't holding it with the correct ammo. I propose all guns to be empty at the very start of the game, for them to reload only by clicking the R button, and for the ammo to be applied based on the ammo used by the last person, something like "marking" the weapon with the ammo of the user that was the last to set it, so it will reload with this ammo type every time until someone changes the "mark" for a different ammo. That's of course a change that would probably take a while, but it seems like the best solution to make gunners viable. For as Myroc said, you're basically giving everyone a free "pipe wrench" in the shape of the normal ammo clip that has quite universal purpose.
I would even go one step further, for even that won't solve the problem for the guns that are very good with only one ammo type (merc,flamer,gat and so on), not to mention the "normal ammo" idea while being really good, it may be denied for the time being, for it'll probably be quite alot of code work.
One of the reasons there's not much purpose for the gunner is that not only engineer can repair AND use guns effectively, but he also can bring buffhammer, which replaces most of the ammo types in their effectiveness, or gets rid of many flaws of certain ammo types, denying the trade-off for its benefits, and that is basically what hurts gunner the most. Why would you want a class that can use different ammo types for different purposes in situations, when one ammo type engie can bring is good 85% of the time, and more powerful than any of the tree gunner can bring if the gun gets a buff?
For that reason I would like to see buffhammer changed for guns. Reduce the damage buff for them, so a buffed ammo will be more relevant in its effective area than unbuffed one, but so it won't overpower the idea behind bringing more ammo types for different situations. At the same time, I'd also like to see the reduction of the amount of hits needed to buff a gun.
Those are my ideas at least. I tried really hard to come up with some kind of ammo that could help resolve the issue of gunners being hardly relevant compared to +20% damage buff, but as Myroc said, people will simply switch to that ammo an an engineer and the issue will stay there. Whatever ideas we could come up with, I think that to balance-out gunners we need some new mechanic changes rather than simply new ammo type.