ok i hve a curious question thats kinda relevant.. planning on sbr'ing a lower i got before the election. was thinking i'd have time to figure out the details on what upper configuration i would go with. but in filling out the form 1 i need to put those details in the overall length and barrel length. how do i get the overall length if i cant legally put it together to measure? especially if i dont have the upper cause i only have a rifle and a stripped lower at this point. curious for suggestions.