The ATF rules changed since I tried to read them several years ago...as they often do...just like braces and short barreled rifles...after being build for several yrs the ATF said all the braced ARs are now Short barreled rifles...and need to be recalled to the tune of millions of dollars in cost to the builder to recall... or the owner has to apply for SBR.
I worked for a computor, robotics, gov contract company ...who decided for awhile they wanted to build AR 15 components. The company hired a specialized ATF consultant to help them get the correct ATF licenses for the company and navigatethe pages of ATF lawyer language laws...he was kept on the payroll, as long as they manufactured the AR components, which wasn't very long...I guess it wasn't a viable product. We only had one manual Hardinge lathe in the whole company...everything is CNC.
These ATF rule consultants are available for those wanting to do gunsmithing or specialized things like just plating or firearms metal finishing, silencers etc..
So if I were going to actually get into gunsmithing I'd check with a ATF consultant to help navigate the rules and laws not only Federal, but state, and city, regulations on what is current law and a heads up on possible changes. Some cities have gun free zones, against gunshops, sales, or gun possession, etc in certain government areas ...like near schools. So Don't open your gun repair shop there, or if your home repair shop is located inside these zones. So many laws and regulations...that continually encroaching and destroying the 2nd amendment rights, and the courts don't seem to mind...and the federal government just added millions to give to states in the last 1.5 trillion ecomomic inflation reduction package to help every state to pass red flag laws...the Federal government paying states to oppress its citizens...will your state take the money....mine did! Plus several new laws to go with it....and 2 more on the books, sponsored by a "Republican"? Helping Democrats pass more oppressive laws "for your safety"...just of how fast laws change...and why I won't do gunsmithing...too risky, with ever changing rules and fees..."just for your safety"...just the way the government wants it no one in the firearms industry.