A firearm going off in an airplane cannot "bring it down." Hint, "Goldfinger" was not a documentary. 

And most of those guns found result in little to no prosecution because they were brought to the airport by idiots who use their carry-ons for other stuff day-to-day and (irresponsibly) don't check them prior to travel. I can't think of a case where the TSA actually touted "stopping a planned crime" in any of their self-congratulatory "innocuous guns/weapons recovered" announcements. Note that it was perfectly legal to carry onto aircraft until 1968, due to fear of skyjackings, yet it wasn't until 1973 that legislation was finally passed requiring magnetometers, i.e. enable actual enforcement, at all US airports.  We had real terrorists flying with guns then, number of planes shot down from inside?

As for water/fluids, the TSA continues to attempt to stop the last bomb, and often overreacts to what is hot in the news instead of taking real measures against actual threats.