If the firearm is already registered as an SBR, it will have to go to an FFL/SOT (Class III firearms dealer that has paid a Special Occupation Tax in order to hold the license) to await approval. same as with a suppressor. A non-SOT dealer may not hold the firearm for you, as they are not a Class III firearms license holder. If the dealer is agreeable, he might allow you to separate the upper and take it home, but the lower cannot reside anywhere but with a Class III firearms license holder until it's approved.

Further, if the seller has to ship the firearm to the local dealer, it has to be approved via a Form III transfer, which literally takes days to approve.

A "Conjugal visit" is a term frequently used to refer to a rural dealer, or co-located at a range that will allow you to at a minimum fingerbang and caress your expensive, incarcerated purchase, or might even allow you to fire it on his premises.

Now you're up on your NFA terminology, go get it!