Please provide the following when creating the Ticket:

Game Version number: 1.3.0.32 and stable
Modded?:** (No)
Currently in both stable and Exp you cannot change ammo type while in the gunner seat. This is becoming a bigger issue as more ammo types get added to the game.

Technically you can change ammo type but it's huge pain and requires you to dump all the ammo out of the vic inventory and reload with the desired boxes which is not at all practical.
I would advocate for an 'ammo type selection' or a 'hold key' over a single button press, as accidentally hitting the wrong key would get annoying specifically in the heat of the moment.