Here's the best answer I've seen on that...from somebody in the NFA business:
The reality is if you have the 5.56 upper you put on the Form 1, or could borrow one, you'd meet the letter of the law by simply attaching that upper first...then removing it and running whatever 300BLK upper you wanted.Building the gun for the first time is a slightly different issue. The potential law you could break in this case isn't an NFA law, it's perjury on a government form. By signing the Form 1, you are swearing that the description of the firearm you are building is accurate. So if you knowingly apply to build a 5.56 rifle with an 11.5", and then instead build a 300 BLK rifle with an 8.0" barrel, you have lied on that form. There's not an NFA regulation that says you have to build it exactly according to the form, but if the ATF can prove that you purposely lied on a government form submitted to them, you could be charged with perjury.
The underlined part above is key. As they'd also have to prove you didn't put the upper listed on the form on that particular lower first as well. (And I'm not suggesting that you build it any other way than what's on the F1)
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