To be fair back when the OTs-33 only required plastic and polymer to be crafted I expected the same result. But sadly the metal detectors still went off even with no magazine in them.
If I remember right some weapons could be crafted without a workbench (back then you used stoves). So you could take some unsuspecting pieces of plastic inside of a mayor meeting, craft a baseball bat and just beat him down when the time is right. Very funny, actually. I think the same could be done with grenade parts.
But no, this will never happen because of a gameplay and rules perspective. In rare cases some already disassemble their guns (stock included) just to save a bit of money, I don't want to search people for Lego parts when they're back alive because the gun they disassembled in light speed didn't drop on the ground.