it would be possible to fake it, as in perhaps giving the player a "load machinegun into truck" action, driving a ways, then adding an "unload machinegun" action, and setpos'ing the m2 into place.
alternatively, you can use the ole addweapon method to simply give the player the weapon. it does of course fire out the player's chest with no m2 in sight, but those two are your only options i think.