Eform 1 filed for SBR in 5.56

I’d like to get a 300 Blackout upper (10.5”)

So 2 uppers in different calibers but would prefer to avoid another lower.

Is it possible?

Yep. IANAL, but my understanding is that the rule is “as long as it can be easily restored to the original condition”, as listed on your Form 1. So, as long as you retain your original upper, you’re gtg.

Thank you

Yes, this is considered a 'temporary modification' and of ZERO concern to you, or the ATF.

It may not mean anything, but if I'm gonna swap out an Upper Receiver, I feel better if keep those 'temporary modifications' such that the receiver has a longer BBL length than is shown on the Form 1 or Form 4 for that particular Lower Receiver/SBR. Of course with multiple SBR's (I got them w/ an 8.5" BBL, 9.75" BBL and a 12" BBL), it's easy to stick to my self-imposed policy and that way, should anyone ever question anything, I'm comfortable knowing it's registered it the shortest BBL length I might be shooting at any given time.

Register your shortest intended length, and then get other uppers. Until someone codifies it in law, you can have unlimited uppers for your SBR. By the time they prosecute you for that, I think you've probably done a bunch of other bad things and they're trumping charges...

I have 5 SBRs now, and multiple uppers for some of them. If I'm wrong about this, well, I've admitted to it on the internet now...

There is nothing in the statue or regulations that prevents this. As to the shorter barrel theory that's not in there either. But if you feel better about it and get one that is shorter than your original F1 filing changing your F1 is just a simple communication to the ATF away. Which is why the idea of prosecution on this is just silly as the ATF will accept corrections/changes to barrel length and overall length.