Been around a while now. Had a play with one once upon a time, quite useful.
Lets face it, it isn't any fun to HAVE to peer around a corner to try and shoot someone who is posted on you, so this solves that problem.
Either way, if one of you is using this weapon, it kinda distracts the guy while your mate jumps out with the shoulder fired rocket, to give him The Good News.
EDIT: Though, looking at the site further. It would appear that they now have a 5.56mm assault rifle version and a single shot 40mm Grenade launcher, so that saves your mate the hassle!