Sorry to shoot you down on this one, Thebonjo, but airsoft guns inside warbirds is a really bad idea, for a number of reasons.
1) safety. Even though it's airsoft, you're putting it in a fast-moving aircraft that's pretty far above the ground. That places a huge area under threat, requiring at least safety glasses for everyone around. If you make the cannons powerful enough to damage another plane, what then? It's certainly powerful enough to hurt a person, and you also have the risk of planes falling, uncontrolled, from the sky due to damage. This is a huge safety risk that I cannot imagine any park or flying field accepting.
2) practicality. Airsoft guns are huge, relative to Flite Test planes, even if you strip off all the decorative plastic and use just the functional guts. I just can't see a plane like the Spitfire or Mustang carrying even a single airsoft cannon aloft. Maybe if you go up to 200% scale, but even then you have issues. Airsoft pellets are very lightweight and slow compared to real bullets, so they are much more affected by wind and the speed of the airplane. You'd have to lead your target by an insane amount.
3) damage. Watch the Flite Test video about the airsoft Kraken and you'll see the problem. Airsoft pellets just don't have the velocity or penetrating power to inflict damage on another plane. Sure you can leave a few dents, but you won't be shooting anyone down with them. And that's the whole point of using onboard cannons, right? So you can shoot down your opponent? Yeah, it doesn't work. If you switch out for a different cannon design, like what Model Warship Combat uses, you gain a ton of weight (ships aren't concerned about weight, airplanes are), and you gain the additional safety concern that your projectiles will travel much farther before losing velocity.
4) spotting/aiming. It would be absurd to try airsoft combat from 3rd-person perspective. The only way anyone has ever tried is with FPV. But it is actually really difficult to spot another plane with an FPV camera. I should know: I fly formation FPV with my brother all the time, and if we get separated, it can take several minutes for us to regroup even with us talking and coordinating our search. Now imagine that difficulty with an opponent who isn't telling you anything and doesn't want to be found.
You aren't the first to think of this idea, nor will you be the last. I just want you to know why nobody actually does it. The people with the necessary technical knowledge also have the wisdom to recognize why it is such a terrible idea.
rip out the airshoft cannon in a tank. and if you have a large enough farmers field to play on, it won't be a big problem, they're airsoft they can't fly many hundred meters, they don't have to mass compared to air friction. The only ones in danger are the pilots, easily fixed with goggles, but since they're probably gonna fly FPV...