What about automatically toggling to the next full secondary ammo load when your current one runs out. Several times I've peppered planes with pea shooters after dropping my DT because I didn't press the "select secondary" button hard enough on my stick, and didn't have cannons armed. It would be nice when dropping bombs if it switched to guns (or if you took bombs and rockets, it switched to the next option).
This would be nice in bombers as well, when dropping from the bombsite, so you can just keep dropping without selecting different loadouts (good for lancs and ju88s, which I like to fly).
Responses? Basically it's an automatic "select secondary weapon" execution when the last item in secondary is used up. If you run out of cannon it automatically will blank out that gun option to the no weapon status.