Rifle's an M40A1, probably the last time you'll see one of these as they've switched over to the M4 stock almost exclusively..cheek-piece is the fairly stereotypical field expeident taped sleeping mat painted along with the rifle.
This is one of those improvised shooting positions...FMFM 1-3B describes quite a few. Iffin' it was me (and it ain't), I'd use this to pop a shot over some of those sand mounds I've seen on the TV. Looks like his finger's on the trigger... but it's a war zone..
As per Croyance's post, all Marines must qualify with the M16A2 every year, shooting 200yds (standing/offhand, sitting and kneeling), 300yds and 500yds (prone). The use of the sling is allowed only for the non-offhand positions, similar to High Power Rifle competition. Unfortunately, offhand doesn't lend itself well to the use of the 1907 sling, so a more stable platform is needed. Trust me, Marines are intimately familiar with the use of the sling, but the bad guys don't tend to hang out in the same place while the shooter gets his support arm strapped in.
S.