So the main cannon has a huge explosive range and is the easiest to use.

The second weapon shoot fast enough that you can almost always get a kill on its cycle.

And the third one has to be aimed after you've found a target with the second, but it shoots through walls effectively.

I regularly get 6+ kills with AC130, and my high is 11, which I've probably gotten a 3 or 4 times. I don't get that many AC-130s, so I'd say that's pretty reliable on ALL MAPS; on a lot of them, the chopper gunner is useless.