I designed some involute spur gears a few years ago on a 3D CAD system and had some SLS rapid prototypes made. We used SLA (stereolithography) back in the 90s, but it was kind of brittle.
The SLS gears ran fine, but they were not near strong enough to transmit any real torque.
Until these 3D printers use engineered firearm polymers, the idea of making firearms this way is absurd. Then, there is the steel component issue you brought up. That makes the idea of 3D printing a gun with current technology practically useless.
How about printing suppressors?