Its totaly staged. How did 10 unfired rounds make it out the right(spent) side of the reciever? Bad primers on all of them? I doubt it. Its not even well staged. Besides if people were being shot it seems more likely that the camera man standing on the position would be hit first.
I don't think that's an issue. It's a canvas belt with missing rounds. I believe with the M2 .50 (which is the same basic design) and even the M-60 you could slap a belt in and start firing without having to align the first round in the belt. In combat, speed would win out if you had enough ammo to waste, which I bet they did. It's been a while, nearly 10 years, since my "headspace and timing" days so my memory could be wrong
The extractor pulls the round from the belt or link, inserts it in the chamber and then the spent casing is discarded from the bottom as the next round cycles. The links, or canvas belt in this case, then exit from the side.
With a metal-linked belt the unfired rounds would likely be laying on the ground to the side, in this case they seem to still be attached to the belt that is feeding out of the side. You can see that some rounds have been fired.
In my peace time service, we always took the time to seat the 1st round in the belt and carefully close the receiver, but then you were always accountable for any unfired rounds so you made sure you burned every one.
Charon
