Well, since he's decked out in military gear and the rifle is sticking out of his coat, I'd say there's not much in the way of concealment going on here.
...and it's just a bad way to carry a rifle, period. If he wants to use it, it'll be awkward. Walking means it's going to bonk into him. It's not actually concealed. Etc.
There are much better options for this sort of thing out there, and it would behoove some artists to avail themselves of that. You can't create a convincing piece unless you know what you're trying to emulate, and knowing is half the battle. Right?
I've seen slings like that, they're meant for cases where you need to rapidly switch weapons (from rifle to side-arm) or to perform other tasks all without losing the weapon. Normally though, once you've finished with whatever you needed to 'drop' the weapon for, you just grab it and go back to doing what you do best.
Of course, reasons for having to switch weapons rapidly vary, such as a malfunction, or you run out of ammo and have no time to swap mags. But either way, once you're in the clear, you usually pick the weapon back up, and you're ready to fight.