I also have saved on YouTube (and my storage) old US Army instructional movie about almost all of the possible rifle/handgun/machinegun mechanics (not traditional shotgun though) and how they're basically made, explained like the viewer is 5. If you want them:
Three parts of the same movie. Building up from this anyone with a reasonable machining garage setup could build and design from scratch the firearm of their liking.
https://youtu.be/orAi33HjWB8
Here, have some more garage friendly DIY setup. Might need some more reinforcement to make it more sturdy but it's basically the way to go
Spez: guy also has at least two more vids kinda follow up expanding the topic a little bit.
There is also the hydraulic method
https://youtu.be/J3pDlKxou6Y
I also have saved on YouTube (and my storage) old US Army instructional movie about almost all of the possible rifle/handgun/machinegun mechanics (not traditional shotgun though) and how they're basically made, explained like the viewer is 5. If you want them:
https://youtu.be/HJnhr08aIJs https://youtu.be/MJGu8prreZM https://youtu.be/-7Tx8OO3JGU
Three parts of the same movie. Building up from this anyone with a reasonable machining garage setup could build and design from scratch the firearm of their liking.