For me, I just wanted one. As someone else said, if we ever get to the point of the feds knocking on my door, we’re well and truly screwed.
I’ve traveled with mine for training purposes, you just have to file the form 20 early enough. So it’s a planning issue.
I specifically SBR’d a “multi cal” marked lower. I know it doesn’t matter, but it’s an aesthetic issue for me. As I recall, you either need to be able to return the rifle to its original registered state, or change the Form 1. So I don’t see any reason why you couldn’t conceivably have multiple uppers, provided you kept the original it was registered with and provided you didn’t run into any constructive intent issues.
Yup, submit the paperwork and get the lower engraved.