I remember reading about two fighter-planes, the XP-54 and YFM-1 which had gun-mounts that were movable in flight.
The YFM-1 had 2 x 37mm cannon that could be pivoted by either the gunners or the pilot if I recall, and the XP-54 had 2 x 37mm that were mounted in fixed positions, but the 2 x 0.50" were mounted in movable mounts. I'm not sure if these variable position sights were mostly designed to simply adjust elevation, or both elevation and azimuth.