So I was just playing around with the ballistics calculator at handloads.com. With a 300 yard zero, M80 out of a 22" gun will be 60" below point of aim at 500 yards. Out of a 16" gun, it'll be 70" below point of aim at 500 yards. With a 300 yard zero, M855 out of a 20" gun will be 30" below point of aim at 500 yards. Out of a 14.5" gun, it'll be 35" below point of aim, again at 500 yards.
This kind of makes me want to get an AR. Maybe something like a rack grade M16A2 clone, except with a free-float tube so I can use a shooting sling. I think I could shoot a lot better with one. They're inherently more accurate, have lighter recoil, way flatter shooting, etc.