I remember that scene....
I would say that would not be classified as a HMG in CMBN terms, but a light/medium MG. The heavy MGs for Americans have a water cooled barrel, and the Vickers if I'm not mistaken had a water cooled barrel too.
The German MG34/42 gets a bit more complicated as the type of mount as well as the sighting system dictates whether its in a light, medium or heavy MG role.
Firing a heavy Vickers or Browning with a water cooled barrel ala Band of Brothers style from a window still hmmmm. There have been a few exceptions -in Guadalcanal there was a case of someone using a watercooled Browning with their bare hands and no mounts, but that was at close range against charging Japanese soldiers. That was normally not the case and expecting any sort of accuracy at distance was asking the impossible when a HMG was not properly setup.
I'm sure there is someone who can go into more detail about all of this.
Perhaps for this discussion there needs to be a distinction made between a machine gun in a heavy, medium or light role-not just whether its crewed served or not.