There is no Penal code that says it is ok for you to build your own firearms. The PC doesn't work that way, everything is legal unless specifically prohibited.
Same thing with required markings, there is no PC that states that a homebuilder must mark a firearm. The law does require licensed manufacturers to mark the firearms that they make, but it does not require the same of non-licensees.
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No posts of mine are to be construed as legal advice, which can only be given by a lawyer.