Pistol is a pistol unless you make it a SBR by adding a stock to make it fire from a shoulder, or a front grip to make it An AOW. To do this you need to file Form 1 with ATF prior to possessing the stock (grip). Once the form is approved, you need to engrave the receiver with the maker name and location. At that point you can legally acquire the stock (front grip) and assemble.
Once a SBR (AOW) always a SBR (AOW) regardless of how you have it configured at any given moment. [unless you make it a machine gun as that trumps SBR and AOW - but that's for another discussion!]
Lawyers are a good starting point, but they don't lose their guns or go to jail if they give you wrong info, YOU do! If in doubt, ask ATF for the info In writing - it's the only way to CYA!
For more details, you can contact the NFA branch of the ATF at (304) 616-4500 or read the ATF NFA Handbook.
A link to the PDF can be found here: