The 3d printer argument is bullshit. Even with a CNC mill it wouldn't be worth it to do anything but make them for yourself, the reason that the big guys can do it is they buy material in bulk.
The part in bold however, is an incomplete thought. Black market guns from the finite supply of legal guns. There is a small but lucrative market in illegally imported guns from the middle east and Africa but you have to know someone unless you are mobbed up or part of the drug distribution network. You decrease the supply of legal guns, the black market share stays the samee.
Now from a replenishment standpoint.
For the most part, and my knowledge of economics is rudimentary, if the supply is reduced artificially, there will be an increase in demand followed by an increase in price. In my estimation this will lead to one or both of the following:
Criminals find out who has guns if that information is available, either thru public records or a careful bribery of a government employee and there will be a lot of burglaries so they can acquire guns to sell to people who can't afford to buy them legally. It will annoy the people who buy illegal guns already as the price will go up but it's business.
Or
A new supply chain will open up. It happened during prohibition, and guns are way easier to make than whiskey.
Before you start laughing there's a thriving trade in black market machine guns in Australia made by enterprising bikers. To my knowledge Aussies have never been able to legally own machine guns but I guess a draftsman, a machinist and a toolmaker got together at the pub one night and said I wonder.