I'm not on top of the current SOTA in 3d-printed guns, but the way it typically was done in the past is that you don't actually 3d-print all of what you or I would call a complete gun.

The barrel will be metal. In designs made for the US market, it will almost certainly be an actual manufactured gun barrel, since gun parts other than the receiver are not closely tracked in the US. In designs for Western Europe, the metal parts will be either milled or things you can buy at the hardware store[1].

The barrel and chamber being made of something tougher than you can get from an FDM machine is basically a requirement for making a gun that doesn't explode in your face when you shoot.

1: Here's an image of all of the parts going into a gun designed to be made in the EU. Per the wikipedia article, the barrel rifling can be added with electrochemical machining https://en.wikipedia.org/wiki/FGC-9#/media/File:FGC-9_Compon...