A criminal does not buy his or her gun in a gun shop. They buy their guns on the street from people who don't care about gun control or what guns do to anyone.
Is it that difficult to create a pistol or revolver through 3D printing? There are thousands of 3D plans of fire-arms available, which can be downloaded by anyone, and manufactured using a 3D printer. Quality ammo will be a bit difficult to obtain, but I am sure that criminals will find a way to secure ammunition also.