This is 100% nothing more than an attempt to ban weapons.
No different than the hysterical hand-wringing over “Saturday Night Specials”, which were nothing but attempts to keep inexpensive firearms out of the hands of citizens.
One, cartridges are metal. Two, I have gone to the airport after simply handling a recently fired handgun and was stopped, and was singled out for an invasive exam. It has happened when I’ve carried a purse I’ve kept a loaded weapon in, too. I now have a travel purse that has never seen a weapon.
So, this is a gun-grabbing load of bull hockey.
You could make a plastic/ceramic gun which fires a non-metallic bullet and is caseless. It would likely only be good for one shot, though.