Just a thought - What if you set up the cannon as an EO Rocket (like a Maverick) weapon station with an EO weapon that has no model, but does have a sustainer effect that looks like a burst of tracer fire? Then you could fire at anything you could target with the "E" key.
Of course, the turret wouldn't actually move, and the "tracers" would want to accelerate straight out from the chopper and then turn toward the target like an A/G missile instead of head straight toward the target. Maybe that can be worked around by adjusting some of the weapon parameters, though...?
*edit* - just read ALL of serverandenforcer's post and see that this piggybacks off of what he already suggested. Still, might be worth a try, though.