The issue with 3D printers is that the vast majority of ones affordable to the average Joe Blow can only print ABS plastic, which is not capable of standing up to the pressures that smokeless powder ammunition will induce. this is why most 3D printed "firearms" are just AR-15 lower receivers since those just hold the pressure bearing components, which are still made out of traditional materials.
There are 3D printers capable of printing metal components, but those are for large scale manufacturers, and are way out of the reach of most normal people.

Far more cheaper and effective firearms can be constructed using 40 dollars worth of supplies from a Home Depot.