You can even legally buy copies of the U.S. Army Improvised Munitions Handbook [1], which includes instructions on creating improvised firearms and other weaponry.

The moral panic about 3d printed weapons is complete nonsense. It's been trivial to make firearms out of hardware store and few practical skills. Back in high school we even made little black powder cannons in the metal shop, raiding fireworks for their powder and fuse and using ball bearings as the shot.

You don't even need steel pipe to make a shotgun, they're low enough pressure that for some types of shot you can even use copper pipe.

[1] https://www.amazon.com/Field-Manual-Survival-Evasion-Recover...