I found that I can load up to 3 static weapons on a salvage truck (you can not airlift it if you do). Then I drive to a location of choice, use the "unload static weapon", walk it to where I want it, and then hit "place static weapon". Fire the canon or mortor until you run out of ammo, then, drive the salvage truck up to where the weapon is and reload it back onto the truck. Now, unload it again...guess what?...you have ammo again.