Curtiss F-type flying boats in WW1 had a Davis recoiless gun mounted
in the nose cockpit. It had to be fired downward because its recoil
counterweight was a charge of shot in a grease matrix fired out the
back end of the tube.
FWIW I understand the initial idea for the 'Puff' series gunships was
the technique of servicing missionary stations in the Amazon with
Piper Super Cubs by flying in a tight circle while lowering a bucket
on a rope. Due to aero drag on the rope the bucket lagged behind and
pivoted on the polar axis of the circle and the ground people could
remove their mail, etc, and place their outgoing mail and requests in
the bucket which was then hauled back up. The rotation was slow enough
to be no problem, and then the curvature of the rope reminded someone
of the trajectory of a gun so - voila!
As for strafing in the F15 with its upward gun - check the -34 for the
sight mil settings and fire further out so the bullets will drop down
below the extended water line. Time of flight will indicate how far
the bullets will drop - it is all solvable with simple trig.
Walt BJ