The difficulty comes in when you disarm everyone but the criminals.
Two issues. First, give everyone on a jet at 35k feet a gun and have them open up on a hijacker and see what happens. Second, I'm certainly not espousing taking guns away from citizens. Not sure where you are coming from.
What do you think would happen? Before answering, I'd strongly advise you do some research on the myth of "explosive decompression" on aircraft.
Do you really have so litle faith in your average citizen?
Fine. It will poke a few small holes in the airplane. The big valve that constantly leaks pressure outside to balance the compressor will have to close down a bit. The noise from the gunshots will be more disturbing than any pressure change caused by bullets actually piercing the aircraft fuselage. Worst case...the oxygen masks drop and the pilot makes a quick descent to 10,000 ft where oxygen isn't required.
Using frangible ammunition complete eliminates any possibility of breaking windows or piercing the aircraft skin. That is standard issue with the air marshals.
I actually payed attention in my aviation survival class. That's the real world...not the poppycock you've seen in Hollywood movies.
And if we let people carry concealed weapons, the streets will run red with the blood of innocent citizens!