Horse pucky. It’s easy to restrict “approved” ammunition to frangible only, where it would not pierce the pressure vessel or have any other consequences with other than soft targets. Besides, catastrophic decompression is not possible with a single bullet hole or two in a fuselage.
Just the common knowledge that other pax may be armed would be enough to insure that any further hijacking attempts would be a thing of the past.
Add in real profiling, and we could eliminate the TSA completely.
Hijacking is so yesterdecade.
The sealed cockpit door required today during flight makes it virtually impossible for a hijacker to capture control of a plane.
The more likely terrorist situation aboard a plane today is to bomb it, taking the entire craft down. Which is why the teeny tiny toothpaste tubes that are all that are permitted for carryons today, in hopes that would be an insufficient quantity of explosive to down the flight if it proves to be explosive.