AH-HA!
i have been on the trail of the elusive extended m-4 sling for some time now.
close examination of many many pics has confirmed it's existense. i just can't get anyone active to talk about it. top secret.
i finaly talked a vendor at a gun show to cut me several 6 footers from the reel of cordura he has (6' is plenty long enough, trust me). my carbine toting friends and i are pleased. it's all the sling you'll ever need.
have tried many 3-point slings, and even made a few myself. they have their place (my fav is the brit one for the l-85). just not on a weapon you may need to shoot weakside.
the hk iterations have tednancy to disconnect themselves at the rear swivel, dumping the butt of the weapon on the deck with the muzzle pointing at your HEAD! very embarrasing.