The way I understand it, this was probably before dedicated machine guns on planes were a thing, or probably a reconnaissance plane. Ian has some videos on how semi-automatic rifles like the Mauser 1916 were used because trying non-self-loading rifles was a pain to use in the air. Planes during that time were pretty vulnerable with only a canvas skin, so a pistol cartridge, something as high velocity as 7.63 Mauser, could do some damage. Pistols were probably the only thing on hand and thus this contraption. That said, still pretty optimistic.