[Tony Williams]

It isn't entirely clear. The following extract is from
Flying Guns – World War 2: Development of Aircraft Guns, Ammunition and Installations 1933-45 by Emmanuel Gustin and myself:

As a practical weapon system the Schräge Musik was introduced by Rudolf Schönert, who in 4/NJG2 had experimented with upward-firing installations on the Do 17Z‑10 and Do 217J‑1 amidst considerable scepticism from his superiors and his peers. These installations were probably never fired in anger, and success remained absent until Hauptmann Schönert became commander of II/NJG5 in late 1942. A Bf 110 was modified by Paul Mahle to carry two upward-firing MG-FFM cannon, which were installed at the rear of the cockpit. Their closeness to the rear gunner must have been uncomfortable, but on the other hand they were easily accessible to replace the ammunition drums. The Revi 16N gunsight was modified to allow the reflector to be placed above the pilot’s head, while the sight itself was further to the rear. (Later the cockpit canopy was given a bulge that allowed an easier installation of the sight.) The first nightfighters equipped with upward-firing guns entered operations in the late summer of 1943, and during Bomber Command’s attack on Peenemünde in the night of 17/18 August 1943 the system finally had a good opportunity to prove itself. It contributed a lot to the big successes of the German nightfighter force in the winter of 1943-1944; one German estimate was that Schräge Musikaccounted for 80% of the British losses. These culminated on 30 March, when Bomber Command lost 95 of the 795 aircraft sent to bomb Nuremberg.

Tony Williams
Military gun and ammunition website: http://www.quarry.nildram.co.uk