A metal detector that has a 99% chance of stopping guns -- meaning some slip by -- would be awesome.
It's a classic problem in computer security that, if it actually works, people say "why do I need security since there are no incidents?"
Metal detectors, even if not completely perfect, stop people from bringing serious weaponry onto airplanes because people don't bother bringing serious weaponry in the first place. The chances of getting caught are too high. People who simply don't bother will never show up in your direct measurements of the effectiveness of the metal detector.
There's a lot of TSA crap and the whole agency should probably be zero'd out, but this isn't the right measure.
If a metal detector at an airport never stopped a gun, that wouldn't mean the metal detector is useless for keeping guns off planes.