Yup, a Rheinmetall Borsig Flugzeugmaschinengewehr MG 15, with converion for ground use.
Edit: I don't see the shoulder-stock adapter. Perhaps it is a standard MG 15 and that it is just used for target practice by bomber gunners. the "4" sign indicates that it is most likely a practice range, and not a defencive emplacement.
... and it has aircraft sights.