It's pretty difficult to tell at what airspeed the 'sideward' flight is done (supposedly to show the ability to point the gun at nearly anywhere). I've see it done at about 60KIAS, but never faster.
Nick could elaborate on this from Commanche experience, but my two cents worth-
The fire control system would have a very difficult job sorting out the windage corrections to get rounds even close to the target initially. It would take several seconds for it to sort out where the relative wind was really coming from, at a minimum.
The second problem is that during the sideways flying, there appears to be a reasonable amount of pitch movement - again a real problem for the fire control system to sort out. Even a small change in pitch is going to make a big difference in the trajectory of the bullets. Roll corrections are a bit less of a problem. Yaw errors also obviously a big problem, but it's not clear if the machine is yawing very much.
Interesting that we never see any footage of the KA-50 actually firing it's gun during this sideways flying, let alone getting rounds on a target.