Holding down left mouse and then right mouse click. This gives instant firing action with no animation and the white dot remains as the cross hair. (white dot sights). There is no sway so you'll always hit the target you aim at.
With the game still running while writing this, I alt tabbed back to play and could not repeat the action with the UMP, But I was able to repeat the action with the AKM. This action can be done while walking, jogging and also sprinting.
Single player offline mode.