Or any other compact side folding PCC that can fire while folded and doesn't require reassembly to fire (under stress and having just been shot down, so probably under fire and already injured as well). The new Ruger PCC in pistol configuration with a brace is one that can be SBR'd and pressed into service, so are the Vector, Uzi Pro and Stribog. As far as flying around with a bunch of ammo on standby, never to be used probably, the 9mm weighs a lot less and packs more compactly than 5.56. Space and weight are at a premium on any aircraftRuger pc carbine would be better.
You're on the right track with keeping the two halves at least bolted together. Being able to fire while folded is where it's at, though. And not needing reassembly after an emergency or some other stressful or catastrophic event trumps any caliber considerations. It's "grab and go" time, not "time out while I fiddle with putting my shit back together" timeI think I'd rather have that folding ar that came out a few years ago. You can't do much with half a gun. At least a hinge would keep the halves together.