FWIW.
There is a .50 cal boresight device. It has a pin that fits into the bore of the MG and has an optical device using a prism that allows the viewer to see where the gun is pointed from 90 degrees away. The scope also has a cross hair so that it can aid in aiming the gun. This type of device is still used today for those systems that use a remote fired .50 cal like the small "min turret" in the M60 tank. This allows the crew to align the optical sights and the gun.
It looks as if the ground crew in the picture are using a device like that to aim the guns on the jug.