I used to carry my M16A2 with a British SA80 sling. I absolutely loved it. Had to rig it a little strange. Used a keyring on 550 cord tied around the buttstock (through the regular sling loop to hold it). Front goes through the sight post. Comfortable and easy to carry, swings up for action or back out of the way. IMO far superior to the tactical slings they issue these days.
They're not real common, but you can find them online.