How does a metal detector really help?
Someone armed walks through the metal detector. Someone comes over to see what the beep is and gets shot to death. Rampage begins.
Or if there is a security guard near the metal detector, he gets shot and killed first then the perp walks through.
Also, think how impractical it is for everyone going in (or in and out) to have to empty their pockets of keys and coins, and take out their mobile phone. Not to mention various jewelry or clothes with metal such as belt buckles and certain shoes etc. How many employees would be needed for such operations at large venues?
You just can't turn every bar and dance hall into airport security without large inconvenience needlessly.
Even if you managed such an improbable feat, you still have the problem I outlined above. Plus then the next killer would just go to large outdoor gathering spots such as Times Square or free outdoor concerts or wherever hasn't been turned into a fortress.
Absolute security doesn't exist.
These mass shootings rarely or almost never happen in countries that restrict guns. There was one incident about 15 years ago in Beijing where a soldier snapped, took his rifle and started sniping at people as he walked down a street. When the armed police and military came out in force he ducked into some wooded area where he was shot to death. Mostly you get someone with a knife, and that causes a lot less damage and is much easier to flee and to disarm. The worst was one guy who stabbed to death over a dozen primary school students a few years ago. But usually with a knife 2 or 3 victims is the maximum.
And so much for the myth that an armed good guy with a gun will prevent mass shootings. In Cali, the first officer to respond went in and got shot up. He's dead now. The highway trooper (armed responder #2) could only pull the moribund officer's body out and wait for the SWAT team. At the Parkland school, the armed officer didn't dare go in and waited for the police and SAT team. In Pittsburgh, 4 officers got shot. Most of these shooters have significant firepower, substantially more than the handgun most police/armed guards carry. And they usually have the advantage of being suicidal.
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