Complex manufacturing of improvised firearms has been practically made obsolete by the commodification of both steel tubing and cartridges. "Pipe guns" are incredibly easy to make, and require little more than a pipe, a cap, and a drill (which can sometimes be omitted as well). Many common cartridge diameters very closely or exactly match commercially available pipe diameters, and the hardware to make a single-shot firearm is ubiquitous in any store that sells plumbing supplies. Pipe guns are simple and cheap enough to make that some people abuse gun buy-back programs by deliberately manufacturing pipe guns for pennies and pocketing the money these programs offer [0]. These are real, functional guns, and I promise they're simpler, faster, and cheaper to manufacture than any 3d printed gun.

0: https://www.thefirearmblog.com/blog/2014/11/17/handing-zip-g...

I assume this is mostly for a shotgun shell affair? otherwise the difference in bore, and particularly the seam that is present in almost all steel pipe (unless its drawn-over-mandrel which is a more speciality product), would make it pretty dodgy to fire a proper round