[olefebvre]

John, AFAIK the engine mounted MG-17 proved troublesome in the 109B for various reasons mostly related to poor cooling. In the later variants the space behind the engine was used for the Oil tank and installation of such a large weapon as the MG-FF would have required a move of the oil tank back in the wing as it was in the Berta. Nonetheless the DB engineers could not guarantee that the DB601A could work properly in such conditions probably because the longer oil circuit and oil tank position would have required more powerfull oil pumps. This lead to the the project being abandonned.
The introduction of the MG-FFM as an engine mounted weapon was put on hold, it was not until the advent of the 109F that this weapon was re-introduced thanks to a rethought Oil system permitting the use of an engien mounted weapon be it an MG-FFM or an MG-151.