Hell, I know how to make a double-barrel, over & under, shotgun from scratch. Literally a visit to Home Depot to get everything needed. And, that's a very short list. I don't create it because I don't want to rot away for years in prison for doing so. But I have the knowledge and the How-to Manual. This was long before the internet existed. Heck, even further back, find an old copy of the U.S. Military Jungle Survival manual from 1965. It'll teach you step-by-step how to assemble a single-barrel shotgun from pipe, scrap wood, and cloth. With an accompanying picture of a downed air-man who did just that. Photo of him squatting down with his improvised shotgun by his side.
Trained machinist with all the tools and a workbench. With the right manual, literally fabricate a fully functional semi-automatic pistol. It's not Rocket Science.