This isn't new, or unique to 3d printing. Plastic/polymer firearms have been around for a long time, including a few out there with ceramic barrels and chambers.
Ammunition is easier to detect, generally - almost all of it is metallic-cased and uses a metallic projectile. There have been attempts to create practical baseless ammunition, but they've generally been failures and I'm not aware of any available for purchase commercially.
Ultimately, this sort of thing is interesting from a technical perspective but not really very useful. This is even more the case in the US, as firearms are ubiquitous. In 2016, there were 14.5 million active concealed carry permits in the US, and in 11 states, no license was required to carry concealed.
Given that something like one in twenty or so adults in most parts of the country are probably carrying a firearm at any given time, knowing "there's a gun in the room" isn't very useful.
> Given that something like one in twenty or so adults in most parts of the country are probably carrying a firearm at any given time
I think that’s a gross overestimation. I would guess that most CCW permit holders hardly ever carry in practice (source: used to have one).
There are states where you don’t need a permit to own a pistol at all. In others, you may not technically need one to take your gun to the shooting range (because after all, if you own one, you should practice) but...
If you’re stopped by police in a context where you don’t need a permit, it’s still always better to have one. If the police aren’t aware of the law or the particular permit exception you fall under, the best likely outcome is you get put in a cage while they figure it out. Having a permit helps avoid that.
If the NRA was clever they would make a much bigger deal about how many guns are already out in circulation in America, and how there's literally nothing that can ever be done to remove them all. The vanguard of gun violence prevention's focus on keeping guns out of the hands of America's growing population of violent mentally ill individuals isn't realistic. It's going to be much easier to identify and remove the violent mentally ill people from society than to remove all of the ways in which they can inflict harm.
But now you're talking about detecting a nail. Which if you decided to ban (somehow?), you'd still be tasked with detecting a tiny object. Most metal detectors are tuned not to even care about normal size belt buckles as far as I know.
> I wonder if there's any type of plastic filament that could do the job.
On a practical note, probably not: the designers probably tried really hard to make a fully plastic gun - they took the trouble to make the barrel out of plastic - but didn't. Stands to reason that it's due to necessity.
At a more theoretical level, I think you'd find it exceedingly difficult to trigger a primer using a material softer than the primer itself.
I'm sure it's not impossible - for example, you could probably trigger a primer by shooting it with a plastic bullet - but within the engineering confines of a handgun it seems implausible, particularly since the easy solution, using a nail, is so readily available.
