I have always used a VERY simple 550 Cord sling on my M16A2 when I was still foot patrolling.
Basically I made a slip loop that fit around the reciever end of the handguards, and attached the other end to the Sling Swivel on the buttstock.
If you are a right handed shooter, you put your right arm and head through the sling. When the rifle hangs, it does so cross-body muzzle down with the stock almost in your shoulder. It only takes a minimum of coordination to keep it there. You can adjust the lenght for patrolling with a ruck or with just deuce gear.
Since it was only 550 Cord it didn't rattle, it was CHEAP, and you could drop the rifle and transition to pistol, or swing the rifle out of the way for more mundane tasks.
I wish I had a rifle to rig it up on and get a pic, but alas I have been without a black rifle since I left the corps in 2000. I am hoping to resolve that in the next week or so. (was waiting for the dreaded ban to expire)