All of this goes out the window because you've not accounted for the fact that we already have more firearms than people in the country, and most of these firearms can last well over 100 years if properly maintained.
The problem you posit here is primarily one that only applies to a ban at the start of it. Presumably any prohibition on guns would involve confiscating the existing ones. I would expect some violent resistance, but the vast majority of people will just hand them over to avoid trouble. Long term it would be much more viable - just look at other countries that have high restrictions on gun ownership.
As was pointed out 3D printed guns are at this time are primitive. The materials available for 3D printing are not sufficiently strong to produce a durable weapon -
example. Even if quality materials were available, the high end 3d printers that could use them are extremely expensive (tens of thousands of dollars) and sold/leased by a very limited number of companies, and as such it would be easy to trace who has those printers and keep tabs on them. Additionally you won't be able to get materials from just anywhere, and government regulation could keep track of orders for materials that could be used for such purposes. There's also again the matter that 3d printing will require technical expertise that most people just won't be able to get.