Phil, I agree entirely with your sentiments re firearms on civilian aircraft. However, the projectile can be modified to cause maximum damage and be contained within the body of the target. Getting gruesome here but... That's why the centre of body mass is the primary target zone, maximum primary damage with maximum containment of fragments. In the recent hostage incident in Sydney, reports suggest that one of the hostages was killed by police bullet fragments from a ricochet off a hard surface within the cafe, not by fragments exiting the body of the hostage taker. In any case, those were not the type of bullets that would/should be used if governments are knee jerked into allowing arm on civilian aircraft.
One further point, I believe I have seen research somewhere ( please don't ask me for a reference or citation, it would take me ages to dig it up) on the effects of multiple small penetrations of a pressurised aircraft hull. I believe the results, while dramatic were not catastrophic. That doesn't in any way change my opposition to armed enforcers on civvy planes.