Let's see: Everyone who does screening. Everyone who poses a natural target (judges, DAs, federal officers, etc) whose protection depends on screening. Everyone who works in a federal building where they are not allowed to carry guns, and therefore relies on a metal detector to keep gun carrying people out. Everyone who flies. One could go on.
You're conflating magazines and guns. A magazine can't fire bullets. It can't hurt people. So who the hell cares if they get picked up on metal detectors? You want to catch the gun, not the magazine.
Plastic 308 rounds. Those are real projectiles, not dummy rounds.
I do not know if the entire cartridge is plastic, though. That primer definitely looks like metal. And I'm going to assume steel for the cartridge body (wow, it was hard to think of another word than 'brass').
Thus are military training rounds. The bullet are plastic, but the cartridge is brass. They are generally not deadly, and are sometimes used to fire at people with (as a form of paintball, but that is ill advised).
They are meant for training with outdoors. They are cheaper then normal bullets, less pollution then lead and don't goes so fare (a normal 7.62mm bullets can be deadly for upwards to 3.5 kilometers, making random shouting in a simulated setting outside unsafe).
The IDF uses rubber bullets extensively as a non-lethal form of ammunition. I think during the Nth intifada there was a story about plastic bullets being used, with them being actually worse than metal since they wouldn't show up on x-ray, although they would shed energy and momentum really fast compared to metal.
Huh, very interesting. Those would definitely be deadly (considering that even "blanks" can be). I think the cartridge body might be plastic too, if I am reading that page correctly (probably not too odd, that is how shotgun shells are often made I think).
Of course those rounds don't have enough recoil to cycle a semi-automatic rifle so I'm not sure how much of an advantage having a high-capacity magazine for those would really give anybody.
I think the cartridge is mostly plastic too, since it says "plastic case cartridge... steel base...." And if it were steel why would they bother painting the cartridge blue? The primer is definitely steel, however.
In any event, the barrel at least has to be metal, so this seems similar to the risk of someone bringing gunpowder into a secure area.
Never head of those before. Very odd. Looks like the whole base is metal, which you would expect, as I'm not sure plastic would be durable/stiff enough for extracting the spent cases reliably.
Still trying to figure out what on earth these things are used for. Seems too dangerous to be used off a proper firing range. What good are they?
That really has nothing to do with plastic magazines anymore does it? I'm sure they don't allow spear guns, slingshots, and potato cannons (not metallic!) in court houses already.
Things that can be picked up by metal detectors easily: bullets, ammunition casings, gun barrels, gun slides, recoil springs, magazine springs.
Polymer cased ammunition exists but is very rare and not general purpose (e.g. flechette rounds), but even those rounds would show up quite easily on a magnetometer based metal detector due to the steel content in the flechettes. The idea of a "plastic gun" is a myth that has been advanced due to the extreme ignorance of the media inducing a moral panic in the 1980s after the introduction of the Glock 17, which merely had a plastic frame and still contained over a pound of metal in it.