I love this part of editing. This was the first thing I messed around with when I learned the in-game editor. Try this...
Add a civilian plane to the map, then in its init field put this...
this addmagazine "Browning"; this addweapon "Browning"
Your Cessna will now shoot .50 cals from its nose.
Unfortunately, you can't really decide where it fires from, but everything works. The Mavericks on the A-10, the TOWs from the Cobra. Everything. Except infantry weapons.
You can actually do a lot with these simple commands. You can give a simple foot soldier FFARs or TOWs as weapons (although it looks somewhat strange, as they use the standard shoulder fired animation, but there's no model). You can even add a Browning 50 cal to a footsoldier (but it shoots from his chest) The only thing you can't add weapons to, for some reason, is cars.
Anyway, hope that helped a bit.