Absolutely it will work with the 20 inch AR!
I got one for my M4, and it is the best sling i have ever used. I bought GG&G's front and rear sling thing and when I recieved the sling, I discovered I didn't even need the sling things. The SOP sling is infinately adjustable and the ERB is a very nice feature should you and your weapon become entangled on brush,barbed wire or whatever. I highly recommend it, it is a great sling!