We used to use M16's set up like that when I was with the SEAL team, back in 'Nam during the Tit Offensive. They had the Bushmaster upper, but mounted on Mattel lowers.
One guy in my outfit used one to take a shot at a Viet Cong sniper we spotted 1250 meters out in the jungle. Well, you know how M16 bullets tumble, right? The tip of the tumbling bullet got hung up in the slot of his flash suppressor and sheared off. He missed the guy he was aiming for, but the bullet tip killed another VC hiding in the dense bushes less than 10 meters in front of us who was about 2 seconds away from wiping our whole squad out with fire from his mortar.
Best damn rifles we ever had.