Me and my mate both text each other with the exact same problem. We both took over an hour to work out that you have to push the barrells down the slope.... Derp.
i think it was maybe half an hour or so if Im honest... tried to sound cool <)
As i feel quite embarrassed i actually stood there for so long with the mounted machine gun ripping up hundreds of Viet Cong, my eyes hurt i was at it so long!
Quite funny now thinking back though XD