Who cares if a magazine can be picked up by a metal detector?
I mean, if you threw it perhaps it could hurt, but if it's made of plastic probably not so much.
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').
Now, a fully plastic gun is another story...
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.
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?
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).
More info about this bullets are available at http://gunlore.awardspace.info/rifledarms/ammunition/dag762b...
However caseless ammunition also exist: http://en.wikipedia.org/wiki/Caseless_ammunition . Thus could probably be made without any metal.
http://www.nytimes.com/1988/09/29/world/us-protesting-israel...
http://www.nytimes.com/1989/01/29/world/israeli-army-lawyer-...
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.
Do distinguished them from other rounds. When I was in the Norwegian military we had at list bullets painted:
Red: Blanks.
Blu: Plastic bullets for training. Cheap and less pollution then lead.
Yellow: Tracer rounds.
Newsflash #2: we've had plastic magazines for years already.
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.