And you can't make a plastic barrel that'll work with the pressures generated by any cartridge.
Oh, you absolutely can make a plastic barrel that will work with the pressure generated by some cartridges, and probably any cartridge. What you cannot do is make it the same size and durability (service life) as a steel barrel.
Depending on variables, it may need to be several times thicker than a metal barrel and the bore may only last a handful of shots but it can be done. its just not remotely practical.
I do wonder about the "fear" of a gun that can't be seen on x-rays...for two main reasons, first, even plastic is "seen" in x-rays, just as flesh is. It appears transparent, but it clearly can be seen. SO, if you have people who are actually LOOKING at the x-ray machine screen, they can see even a plastic gun. Whether or not the people recognize it is a different matter.
Second, anyone ever come up with ammunition that doesn't show up in the x-ray?? Or metal detector??? Not that I'm aware of. Even with plastic cases, the bullet is still the bullet, lead is kind of clearly seen in x-rays, and plastic bullets aren't very lethal compared to regular ones.
So, even if the gun gets through x-ray without detection (and that doesn't actually happen, its media BS) the AMMMO is detectable, and without ammo a even a plastic gun isn't very dangerous.
Terrorist weapon? ok sure, if you could 3d print an entire AK and its ammo, for less than the cost of real metal ones, on the black market, MAYBE...
OF course, better outlaw 3d-printed box cutters too!!