[night reader]

...I remember reading a thread, I think it was here, shortly after 9-11 where a pilot, who also carried, played down the danger of shooting a gun in a plane, possible puncturing the outer skin. Anybody remember this?

Yep! Like you, I believe it was here. If I recall correctly, it was asserted that in a large modern airliner, there is already the equivalent opening of over a square foot to the outside and the pressurization equipment was capable of compensating for this a the much smaller opening that a bullet might make. The writer also asserted that control lines, whether electrical or hydraulic, have multiple redundancies.

Again, if I recall correctly, that poster claimed credentials which would make such a claim credible. I, myself, make no such claim.

[Puppage]

If I recall correctly, it was asserted that in a large modern airliner, there is already the equivalent opening of over a square foot to the outside and the pressurization equipment was capable of compensating for this a the much smaller opening that a bullet might make

You are absolutely correct with regard to the "1 square foot" premise. A bullet hole in the fusalage would do nothing like you see in the movies.

That being said...ARM THE PILOTS. What's worse.....defending a small cockpit door, or a sky marshall shootout in coach? IMHO, this is a no brainer.