Watch the video of the shooting in slow motion, you'll be able to see that the shooter who cap the RGP shooter had 1 of his shot hit the road surface before hitting the skull of the RPG shooter. Clearly its a 5.56 round that hit the taget, 7.62 would have gone through his head. A 0.50 cal would have bust the targets head( there are plenty of pictures showing a head shot with a .50 ).

I assuming that the 5.56 shooter was at an extreme range without scope, he was estimating his shots by firing as many rounds as close as possible to the target & luck was on his side when that 1 round bounce off the road surface & hit the target.
