[Salgak]

If she wanted to do it right, she could have pulled the bolt and cut both the upper and lower receivers, doing it legally. . .

[Myrddin]

She could have removed the AR upper from the lower and placed the lower in her safe. Cut away all you want on the standalone upper, but never assemble it back on the lower. An AR pistol assembled with a lower that has a non-stocked buffer tube is fine. Put that pistol upper on a lower with a rifle stock and you have an SBR. The lower must have an ATF Form 1 or Form 4 with a tax stamp as well as matching the length and caliber of the upper if it is under 16 inches. You should always keep the Form 1 or Form 4 that authorizes the assembled SBR when in your possession. A Form 1 SBR lower will have the firearms trust name, city and state engraved if it is part of a trust. I've never done one as an "individual", so I'm not current on engraving requirements for that configuration.