I know if I buy a SBR I can swap different uppers. Does that include if it’s registered as a 5.56 and I want to put a 300blk out upper on it?
Lamented, there is an “NFA” sub-forum where you may get very knowledgeable feedback.
The quick answer, however, is yes, you can put different caliber uppers on the SBR.
Yes…doesn’t matter what caliber or barrel length you put on it. Note that if the barrel length of the upper you’re using is greater than 16 inches it’s not an SBR and not subject to the NFA no matter what the registration status of the lower is.
I have seen guys register there lower for multiple calibers too. That’s a good idea these days since so many people run 300BO. I’ve never done it.
PB
I wouldn’t put multiple calibers on the Form 1/Form 4, just as I wouldn’t try to register for multiple barrel lengths. If, after building the rifle, you want to make a permanent change to its caliber or barrel length then send a letter to NFA branch telling them that (apporval not required). If it’s not a permanent change of either then I wouldn’t bother.