The hole that 2# part screwed in, is for building the channel for air, which will push magazine index assist pin, finally get the rotation of magazine
Thanks for your reply. From the diagram, the end of the screw #2 inside the action is not resting on anything so it seems to me that the screw is just there to plug up the hole to stop the air inside the action from escaping via that point. Is this what you meant by "building the channel for air" ? If the screw is removed, I believe the only thing happen is degassing of the action hence disabling firing or actuation of the magazine. If the screw is removed for long enough, the air tube will ultimately be degassed as well. May I know if this understanding is correct ?