I nominate this: https://github.com/maduce/fosscad-repo/tree/master/Firearms/...
If you want a gun, your best bet is probably to go and buy one.
This is why some guy was able to 3D print part of a Glock.
As for assembling one, anybody who does proper gun maintenance takes their gun apart and puts it back together.
There's a market for DIY guns (outside of the 3D printing scene) where they ship you all the parts.