This is one of a few related properties. These are:
ObjectTemplate.SetNumberOfWeaponIcons # ObjectTemplate.SetPrimaryAmmoIcon "Ammo/Icon_*.tga" ObjectTemplate.SetPrimaryAmmoBar ABTypeName ObjectTemplate.SetSecondaryAmmoIcon "Ammo/Icon_*.tga" ObjectTemplate.SetSecondaryAmmoBar ABTypeName
These all have to do with the ammunition/overheating status display for each weapon, the primary (left mouse fire) and secondary (right mouse), if any. SetNumberOfWeaponIcons specifies the number of weapons that have a display on the HUD, a value from 0 to 2. SetPrimaryAmmoBar sets the type of display needed for the weapon, similar to how handweapons use this value in SetHudAmmoType. Note that the defined names are different, though, starting with AB instead of AT. Here are the values used for vehicle weapons:
ABNone - for positions that have no weapons; set for vehicles without weapons ABAmmoBar - for weapons with no reload wait time but can run out of ammo, like fighter plane guns ABAmmoBarHeatBar - for weapons that overheat and run out of ammo, like machineguns on tanks ABAmmoBarReloadBar - for weapons that need time to reload, like tank or ship guns ABHeatBarOnly - for weapons that overheat but never run out of ammo, like stationary MGs ABReloadBarOnly - never runs out of ammo but wait to reload, e.g. destroyer and battleship guns ABIconOnly - never overheats, never runs out of ammo, never needs to reload, e.g. flak guns
If SetPrimaryAmmoBar is any setting except for ABNone, the SetPrimaryAmmoIcon property is set, to define an image of the type of ammo used (bullet, rocket, etc).
See the HUD Icon Tutorial for more details and examples.
Source and more details