When buying a SBR rifle can you take home the upper before the stamp comes back, because the SN is on the lower?
If you're referring to an AR, technically yes, but if you have another AR at home it gets muddy. Some would construe it as constructive intent.
I think they did away with the constructive intent. I think.
No, they didn't, because there was never such a thing as constructive intent.
What you're thinking of is constructive possession. It's basically a legal doctrine that states that if you have all of the parts to make an illegal weapon, and no clear LEGAL use for those parts, they can charge and convict you on having the illegal weapon.
When people say, "They did away with constructive intent (possession)" they are referring to a recent ATF opinion that was a follow-up to the US v Thompson-Center case where the Supreme Court ruled that having the parts for a Thompson Center Encore both to use it as a pistol and a rifle was not illegal.
The follow-up ATF opinion basically stated that if you had a firearm that started out as a pistol and it was a kit or modular-type firearm, you could convert it from a pistol to a rifle and back again, as well as own the parts for both configurations and it would all be legal. If your firearm started life as a rifle, it could NOT be converted to a pistol and back.
The key is that you have a legal use for all of the parts, and your firearm started life as a pistol.
In the OP's case, if you bought an SBR, you COULD take the upper home while you're waiting on the paperwork. However, if you own other AR rifles and do NOT own any AR pistols, you could still be charged with possession of an unregistered short-barrel rifle, as you don't currently have ANY legal use for that short upper. If you have a pistol lower, you would be OK. If you have no other AR rifles you would also be OK.
Basically constructive possession means that you have the parts to make an illegal firearm, and you have no LEGAL use for those parts.
Another example would be having an AR and all of the FCG parts for an M16. You have the parts to make an illegal machinegun, and no legal use for those parts. Now, if you had a legal, registered M16, you could have extra parts laying around as spares.
And we see clearly how stupid the law is.
You have two identical stripped lowers. One has a magic wand waved over it that says "Pistol" and the other has one that says "Rifle".
Own the first and a short barreled upper (unassembled) and you are fine. Own the other and a short barreled upper (unassembled) and you go to jail (and yeah, I know most stripped lowers are never assembled prior to sale, so this generally isn't a concern).
Of course, the only way to know the difference is to back track the serial numbers of the lowers back to the manufacturer to figure out which magic wand they waved over them (or if they wave no magic wand at all, and it's in the "white"), unless they engraved the words "pistol" or "rifle" into the receiver. But even then, that doesn't matter if they went into the manufacturers books the other way around.
Ridiculous.
Manufacturers don't designate a stripped lower as a rifle or pistol unless they are built as such. They are receivers, plain and simple. ANY stripped lower can be built as a pistol until and unless it is built as a rifle.
And we see clearly how stupid the law is.
You have two identical stripped lowers. One has a magic wand waved over it that says "Pistol" and the other has one that says "Rifle".
Own the first and a short barreled upper (unassembled) and you are fine. Own the other and a short barreled upper (unassembled) and you go to jail (and yeah, I know most stripped lowers are never assembled prior to sale, so this generally isn't a concern).