Simply press 1 and 2 on the second tankseat (Brenodi APC, Light Tank ...). You will notice, that 1 gives you the secondary weapon (pistol) and 2 the primary one. This is reverse to the normal case outside a tank. Its not a big "bug", but i think the order of weapons should be kept in the same way everywhere. In addition to that, the crosserhair for the handguns on the second tankseat are in a way displaced as long as the tank is positioned on a surface with an angle of climb ‡ 0! So the crosshair of your handguns is simply useless, as long as your tank is driving up/down a hill, or is just standing on the way up/down.