 CONT: When Jack catches up with Cheng on the roof top of the  industrial building he tells him to put down his weapon. The breech  block of Chengs gun is pulled back since he is out of ammo. In the  next scene Cheng places the gun on the metal bridge with the breech  block in its normal position. There is no sound or action that would  prove that Cheng released the breech block of his gun.