I'll try the M1917 for taking out planes .. I usually get that one for what I call "suppressive sniping", never used it to down a plane.
The Parabellum I have tried now, and it could work, if the plane is damaged already, and the pilot not very good. So, that doesn't happen a lot, since the bad pilots usually get killed before they even can attack you. And the good pilots, well they kill you.
Camping close to an AA gun seems the best option, and try and hit damaged planes, either with the Parabellum or the M1917. Somebody suggested the MG15 as well, which is a gun I like a lot but haven't used a lot to hit planes ..