It depends on the level of 3d printer you're talking about. Your average $300 at-home printer is basically the hot end of a glue gun on stepper motors, though you might be surprised at some of the materials they're capable of printing in. Everything from basic ABS plastics to Nylon and Carbon-Fiber reinforced filaments are easily available.

If you're talking commercial grade, $10k+ printers, that's an entirely different story. Commercial printers are capable of printing objects out of steel. There's been a lot of work in that area to print all kinds of things from guns parts in military grade polymers to entire engine blocks, no assembly required.

On the 3d printed gun end, supposedly people have figured out the issues to the point where you can print 100% of the parts out of super basic plastic (the most commonly used plastic in 3d printing is PLA, which has a melting point around 200 degrees Celsius), though the stuff I've seen online is more about using internals from cheap guns and 3d printing the external "furniture" of the gun either for custom cosmetics or aftermarket parts like handles and grips, or to create an expensive gun out of cheaper components. As for the ammo, I've only heard that "people were working on it." I don't know any of the specifics.