You didn't mention if it was a transferable receiver or not. If so, build it. If it is not a transferable receiver, like a CATCO "tube kit," only a class II manufacturer can build it and it will not be civilian legal.
"Those who live by the sword are probably pretty f***ing good at it."
"Instructions for a successful gunfight: Front Sight, Press Trigger, repeat
as necessary." - B. Braxton