Concerning mounted on the ground or on a plane I don't really see a problem as showcased in this video of the GAU-12/U used in a gun pod on an AV-8A.
From the video:
- on a fixed mount on the ground: design specification 5 mil, achieved 3 mil
- on the airplane: design specification 7 mil, achieved 5 mil
This shows that
a) the ground and plane mounted accuracy doesn't deviate by much even when fired from a moving aircraft (and this is pod mounted, not mounted inside the fuselage with better center of gravity concerning recoil)
b) a gatling gun is very well capable of being *more* accurate than design specification
So I would expect the same (i.e. not a lot of difference) for the M-61, too.