Is it legal to copy a product and reproduce it and sell it.?

I'm getting into gunsmithing and as being a machinist, I can easily copy and reproduce things. I was wondering what I would have to do to legally copy/recreate certain rifle (besides FFL and State legislation/restrictions). Being a little more specific I'm going to reproduce Russian military rifles.... show more I'm getting into gunsmithing and as being a machinist, I can easily copy and reproduce things. I was wondering what I would have to do to legally copy/recreate certain rifle (besides FFL and State legislation/restrictions). Being a little more specific I'm going to reproduce Russian military rifles.

P.S. I'm a law abiding citizen, never been arrested, or convicted of a crime, and have a concealed carry permit.
Update: Side note, I would like to know how many companies can make ar-15's also I have no idea about copyrights of soviet/russian military rifles.
Update 2: On that additional detail, I meant, I would like to know how *so many companies can make...
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