So then according to that logic, then it would be okay...Not sure I totally follow your question…but as I understand…the upper portion of the rifle does not constitute a firearm or in this case the SBR.
Gotcha! Thanks for the advice!You can buy or build any upper you want, currently.
But you need a lower that has been Formed, tax stamped, and engraved before you can put them together.
Pistols excluded, but remember, once a rifle....always a rifle.
Technically, to avoid any chance of running afoul of the "constructive possession" laws, you shouldn't have any sub 16" uppers unless you have at least one SBR lower.
Again, pistols don't count.