When you get the lower in have your ffl register it as a pistol so you can use it as a pistol and a rifle.
I AM NOT A LAWYER, so don't take this as legal advice, and do your own research:
The way I understand Federal law, all stripped virgin lowers have to be transferred as other/receiver and from there you can build it into a pistol or a rifle as you wish. If your FFL transfer a stripped virgin lower as anything else, they're doing it wrong. Get a copy of the 4473 for your records.
If the receiver has even once been built into a gun (barrel and maybe stock attached), it remains what you made it into and must after that be transferred as such.
A pistol can supposedly always be made into a rifle, but a rifle has to stay a rifle. Be careful, it's very easy to accidentally be in possession of an illegal short barrel rifle, even if you simply own parts that can only be configured as an SBR. Like owning a pistol and a buffer tube with a collapsible stock in a box in your garage, but no long gun upper.
Between Federal law, ATF regulations, and local laws, there's a lot of grey area to building an AR pistol, so make sure you really do your homework first.
All that said, they're a ton of fun
