Not sure if it's been suggested before, but maybe allow gunners (or everyone) to switch out ammo instantly, with maybe a 3 second cd before it can be fired again. Switching ammo wouldn't trigger a reload, in this case. Probably doesn't make a huge amount of sense from a lore perspective, but it would let gunners switch between ammo types a little more effectively. It would let them choose the most effective ammo more quickly, which could increase their desirability.
Alternatively, maybe let gunners choose which ammo gets loaded into a gun by default. By that I mean, gunner chooses an ammo type for a gun and reloads it. The 'normal' ammo on that gun for everyone else becomes the special ammo. When it reloads, it reloads with the ammo the gunner chose. Or, instead of replacing the normal ammo, the special ammo could become an option for anyone using it (1 would be normal ammo, 2 would be the special ammo someone else loaded into it, 3-5 would be whatever ammo you're carrying).