Why you can not insert anything from below while the bolt is locked in the rear position?
Empty chamber with the bolt locked in the back.Here the first problem arises:Diagram 1 (from World of Guns): Positions of the shell carrier and shell stop when the weapon is completely empty.Diagram 2 (top down): Back end of the shell stop is on top of the shell carrier (red box), locking the carrier down; pressing the bolt release (blue box) will move the shell stop away and unlock the shell carrier, only then is reloading from below possible.
