Thinking of popping my NFA cherry.

[Irving]

I was thinking 5.56 with an extra upper in 300BLK, is that a no no as far as SBRs go ?

I believe the idea is that you must state a caliber when you file. After that you can change uppers (calibers) as long as the original caliber is still available. It's not necessary to officially notify the ATF unless you permanently change the caliber (file with 5.56, install the 300blk, then sell the 5.56). However, I also still have my NFAHymen, so you better consult with one of the easy girls on here that are also not lawyers, before you take my advice.

[brutal]

That's fine. Only a permanent alteration of length or caliber where you cannot return the lower to as stamped configuration requires any formal notice.

I've both cals and swap uppers around at will.

All my lowers are marked MULTI but were filed with a specific caliber. Actually, they're all filed 5.56 except for one 300BLK.

That "extra" upper needs a legal host, or no available lower in your possession. That's why I recommend filing an existing or new lower and keep that pistol lower around... That whole constructive intent dilemma.

If you have more short uppers than legal lowers (SBR+pistol) you could have an issue.

If you're going the trust route, I would probably just buy a new lower in the name of the trust. Otherwise, you're going to have to do a BGC to yourself when you do the assignment to trust on that gun. In either case, there's a BGC involved.