[ILLusion]

That screw is actually there to make sure the outer barrel remains properly oriented against the upper receiver. The grub screw should stick out of the outer barrel and should fit in to a slot of the upper receiver's barrel entrance. It keeps the outer barrel from twisting when installing the barrel nut. Otherwise, if the outer barrel twists, then the inner barrel will twist as well and the hop up effect will cause your BBs to hook left or right.