There are firearms related .stl’s that can be found, but they are accessories rather than the actual parts that make a firearm function. But there are forums that do discuss and build 3D printed firearms that can be found with some searching.

I’ve done a couple of designs, shims for a Winchester SX4 shotgun to fit several guns to their owners for target shooting and hunting. And an easy loader for my P365XL. That 12 round double stack single feed clown car of a magazine is hard to load without it. But I haven’t bothered to publish either design and I have never wanted to either.