Sheesh. For $12K-$14K, You can buy a Lathe, a Milling machine, a Shop Press, some tools, some AK47 Kits and Ar15 kits, and use your Garage as your shop. Apply for the FFL07 SOT by sending away for the papers from the ATF, set up your appt to have them come by and check out your set up after you have filled out and turned in your application. Let them make sure you're 5x5, pay your $650 a year, and make the F/A Guns and suppressors you want, as long as you send in the proper papers and reg them to your business, you should be GTG. YOU HAVE TO SEND OUT(or fax)and KEEP THE PROPER PAPERWORK FOR EACH FIREARM AND EACH SUPP YOU MAKE ON YOUR OWN.
I have a local friend who is an FFL07 SOT. He constantly makes his own suppressors, mainly because he is in the process of R&D on his own line of Supps. And he converted almost all his assault rifles to F/As after his FFL07 SOT permits came in. He had to send in papers for each gun to the ATF, he had to do the conversions himself and buy the parts to do them himself, but being an FFL-07, you can do that. He also builds guns for customers, you have to manufacture them...you can't just get your permits and make all your guns into whatever you want and buy whatever you want...it has to be a working business. But you can build guns as a side job. As long as you are making SOME income from it, and keeping a VERY DETAILED LOG of your work and transactions, youre fine.
I am ultimately planning on manufacturing Firearms myself. I have lots of ideas and R&D projects involving that side of things. I already make knives and such, I already have 60% of the equipment I need. And I am never gonna pay $12K-$14K for one little trigger group. You gotta be crazy or rich. I am neither....yet.