[RemMax]

Not unless you are a Class 2 manufacture but then if you were that you would know this information!
With regards to the fines and jail time our best best is to go to a Class 2 Manufature or a Class 3 Dealer and talk to them, they are a little more forgiving towards ignorance than the BATFE is but still yet you do not want to ask them how to build a back yard MG like you are asking here or they will likely THROW you off their propperty, not to mention that they are no more likely to tell you how to "build a Back Yard Machine Gun" than I am! I will point out however that if you spend the money and buy any registered Part be it a LL< RR< or a DIAS you can use it with any barrel length you want since a Machine gun is not restricted to barrel length's like a rifle is. Agaain talk to a Licesened Dealer and they will be able to help you with al the info you need to stay legal.