A metal box with a spring in it. Amazing what liberals are afraid of. If they ever succeed in doing this, I guarantee that 20 million people will take up a new hobby: garage machinist.
Already the case in California where they have "build parties". Apparently, in the Gun Control Capital of the USA -- if you build it at home, you aren't required to register it.
Since California passed its Assault Weapons Ban in 1989, the residents have constructed millions of AR and AK type rifles.
All of those are off the books. All of them are quite legal.
Now, expand that to all 50 States. Not going to go too well for the Gun Grabbers, is it?
Or 3-D copiers