I'm pretty sure the reason why gunners don't have tools that change gun stats outside of ammo is for balance. It's so easy to break the balance of guns by even giving them bigger arcs.
If you read the other post regarding gunners, you will see that I am of the minority who think that gunners are perfectly fine and need no buff to make them useful. I say it in the nicest way possible, but there is a huge difference in a good gunner vs an average gunner. And the ammo he takes is a direct extension of what gun is his.
I believe I put a list up in that thread as well showing that all guns can benefit from multiple ammo types besides just the one an engie can bring. The key to remember is that regardless of the role of your gunner, you want him to always be shooting. The gunner role will serve you much better than an engineer in that position.
