Depends on the weapon, I'm sure you could find AR, AK, 1911, or Glock schematics fairly easy.
I'd look into Professor Parabellum's work. You'll find schematics for the Sten gun and many other homebrew options such as Shotguns, SMG's, or Rifles. idk if you'll be able to find accurate specs for many mainstream weapons though, as the only manufacturers aren't likely to share their schematics and processes openly.
So, if you're not into homebrew stuff, id do what the other guy said and learn the gunsmithing trade because you're not getting anywhere with youtube videos or deep web forums lmao
