Real guns aren't hard to get in the US, generally. Much easier than putting together 3D printing and so on, as well as cheaper, and easily made just as untraceable with a little grinding. Most crime guns long ago lost trace-ability - many are simply stolen themselves, anyway. At least if you believe the FBI, difficult as that is becoming. Or surveys of criminals in prison for violence.
Would you rather have some wacko using a plastic gun as likely to blow his own hand off with the first shot, sending a bullet weakly in some direction almost but not as intended, or a real gun that can accomplish a mass shooting?
I want all my attackers to have a plastic gun useful for somewhat less than a single shot. I'm in a lot more trouble if they use a good one, no?
And you can tell the agenda - an administration simply does nothing to violate the constitution - no constructive action of any kind - but somehow this thing is their fault!
Try stuffing crypto back in the box. OK that didn't work, lets have a war on poverty. Um no good, lets try war on drugs - that'll make them unavailable, and no one will shoot anyone over drug turf, since they have so many other means of making a living /s. See the pattern?
A decently working shotgun takes a piece of pipe, a cap, and a nail, if you want to get fancy, you can add the rubber band, and the only secret is the diameter of the pipe...and lots of people know that, or chemistry of explosives or whatever other nasty - if you have the will, you can find a way to do bad things - the will is what needs addressing.
This is ignorant virtue signalling that will have - if it works - a negative effect on the problem, lucky it won't work.