Fwiw, I think the guns used on the RC Combat ships have too low a muzzle velocity to be at all effective in air-to-air combat. I could be wrong though. After all, they have strict rules about having very thin balsa hulls, and the spectators and captains don't seem overly concerned with BBs skipping off the water in their direction.

I do think effective guns on an RC plane is just asking for trouble, but if you are careful, you might be able to manage the risks. My biggest worry would be getting shot while flying one of the planes. Spectators and anyone near by as well. The BBs have to land somewhere eventually. Beyond that, I'd worry about taking hits to the radio gear and getting a fly away situation.

In streamer chasing combat, most of the time a mid-air hard enough to take out the Rx or battery will also cause the plane to go more or less straight down, or in a lawn-dart-style arc. This is because the act of hitting another plane take away a lot of the energy involved, stopping (or nearly stopping) the plane. A BB cutting a wire isn't going to do anything to slow the plane down, so it's going to keep going straight and fast until something happens.

If you're going to do this, be really really really sure you are way out in the middle of nowhere. Personally, I wouldn't bother, too dangerous and expensive for not enough fun.