I didn't say it would be accurate but a faceful of CRV 7s plus 30 mm cannon would certainly deter all but the most eager FJ looking for an easy target.
Yes it would be a head on shot but that's what we used to do against the Lightnings since their cannon were mounted about 2 degrees nose up and it forced them to shove the nose down to get a solution.
However, as previously mentioned, providing you can see/detect the helo in the first place then missiles from a longer range are much easier for the kill.