NMI?(need more info)
You are building an AOW? That will run you a $200,"making tax", to be marginally involved,"in the business of making NFA firearms". If you buy an AOW from a class 2 sot or manufacturer it is only a $5 ,"transfer tax/fee". If you are still going to do it yourself, FILE ALL PAPERWORK FIRST, ONCE APPROVED AND PAPERWORK IS BACK IN HAND(PARTICULARLY THE TAX STAMP), THEN YOU CAN CUT THE BARREL DOWN YOURSELF...Should take a few months from what I hear.