Errm guys, hate to interrupt this advert for firearms training, but isnt there the little problem of explosive decompression if you start introducing firearms into basically a pressurised cylinder..??
Unless you can be sure that all guns have extrmely low velocity ammunition, I can see more aircraft being lost through what the British Army call 'Negligent Discharges'. As for the idea of the Co-Pilot "covering the door"....jeez, this sounds like something of which Holywood would be proud.
Before you wonder by the way, I am a gun enthusiast - but on the ground!