If you have a drill, a hacksaw, and $30-$40 of hardware, you can make a single shot pistol. It doesn't even require any specialized knowledge. For additional $20-$30 and a moderate amount of knowledge, you can make an (admittedly low quality) open bolt SMG.
And of course, this is buying that hardware in US stores. In Brazil or the Philippines, there are entire illicit industries devoted to making firearms of varying quality.
In the end, implementing effective gun requires two things:
1. Killing off everyone who has enough knowledge to make a gun on their own.
2. Destroying all the tooling and hardware usable for making a gun.
And then, after we've returned to the stone age because we destroyed all our industry and killed everyone who had the knowledge to rebuild, we'll finally be free of the firearms menace.