An acceptable false negative rate really depends on the consequences of getting caught.
If someone's nefarious plan depends on smuggling a gun in, they want to be confident they won't be arrested or shot at the entrance. Even failing to detect 20% of firearms means there's an 80% chance they'll be caught before they can do whatever it is they plan on doing. This is also why it's important to have armed guards alongside the scanners. Scanners aren't very useful if the only armed person is the bad guy.
If the consequences of getting caught are negligible (as is the case for anyone trying to bring a box cutter through airport security), then the attacker can try as many times as they want without issue. Even if the false negative rate is low, they only have to get lucky once.
Annoyingly, I can't find any info about false positive/negative rates for various scanners. There doesn't seem to be the equivalent of Consumer Reports or Underwriters Labs for scanners. My guess is that the numbers must be pretty bad if companies aren't willing to go through public 3rd party testing.
If someone's nefarious plan depends on smuggling a gun in, they want to be confident they won't be arrested or shot at the entrance. Even failing to detect 20% of firearms means there's an 80% chance they'll be caught before they can do whatever it is they plan on doing. This is also why it's important to have armed guards alongside the scanners. Scanners aren't very useful if the only armed person is the bad guy.
If the consequences of getting caught are negligible (as is the case for anyone trying to bring a box cutter through airport security), then the attacker can try as many times as they want without issue. Even if the false negative rate is low, they only have to get lucky once.
Annoyingly, I can't find any info about false positive/negative rates for various scanners. There doesn't seem to be the equivalent of Consumer Reports or Underwriters Labs for scanners. My guess is that the numbers must be pretty bad if companies aren't willing to go through public 3rd party testing.