Yeah the AC-130 lasts way too short.
But I found that if you switch between each of the 3 weapons on it you get more kills. So like shoot the big gun, switch to the heavy machine gun and shoot that, then to the little one and shoot that, and repeat. The whole reloading shit doesn't affect you as bad that way.
The AC-130 is way harder to shoot down because it's so high up and it's two bigger guns have the potential to get tons of multi kills. Still, over time you'll probably have more success with the Chopper Gunner, it more consistent.