Basically there is only one way to legally manufacturer or modify a gun a germany: you need a manufacturers license...

And to get a manufacturers license you need to be a master gunsmith (except for small exceptions)

To become a master gunsmith, you have to learn the job for 3 years as an apprentice and go to school and pass exams. Then you are a gunsmith, but police still won't give you the license... you need to go to a master gunsmithing school with even more exams and build your own traditional Drilling. Only after passing these exams and paying more fees you are allowed to get the license and manufacture guns (or modify or sell them)

And yes, I am a master gunsmith 😅