There is not a more dangerous weapon to have onboard an airplane than a gun. Discharged at high altitude, it would cause immediate decompression and a dangerous situation for all the passengers. It's possible that hundreds of loaded handguns could be making it aboard commercial flights every day.
Last week, Transportation Security Administration baggage screeners unknowingly permitted the vice president of the New Orleans Hornets, Joshua Richardson, to board a flight to Newark, New Jersey, with a loaded .38 caliber pistol in his carry-on baggage. It was not discovered until he attempted to board a flight back to New Orleans.