Quote Originally Posted by lonestardiver View Post
Look on the form 1 under change of description. If you sell or trade or no longer have the original upper and you are using one of a different caliber or length you are making a permanent change. If you keep the original and swap out uppers as desired but have the ability to revert back to the “dimensions” as noted on the form 1 then everything is OK. I believe the ATF wants to know of “permanent” changes to a registered NFA firearm.

Item 4 is the description with parts a-i. Those include caliber, barrel length, etc.
There’s nothing permanent about it. If you can change uppers by simply pushing two pins, that’s about as far from permanent as you can get.