Many times the player will find it frustrating that inside a turret, he cannot pick which belt to load. Perhaps I want tracers, or I want to swap out my API for IAI. Not currently possible. You get what you get. It's possible to inspect your own infantry weapon and switch belts, why not a similar system for turrets? First person, hold r and look at the belt. Or just look and interact with the belt. Seems too simple and too necessary to have been missed, so I assume there's some tech reason it's not been added yet.