From US v. Burning Breast:

The relevant evidence in this case came from one ATF expert witness. His testimony was based on ATF records that traced one serialized part on Burning Breast’s gun: the lower receiver. [..] Given these facts, there are two ways the Government could get a conviction. First, it could have proven that the lower receiver found on Burning Breast’s gun is a “receiver,” and so a “firearm” as a matter of law. 18 U.S.C. § 921(a)(3)(B). Thatwas what the ATF agent repeatedly told the jury, and that was the Government’s theory at trial. [..] There is just one problem: an AR-15’s lower receiver does not meet the Government’s own definition of a “receiver.”

To be a “receiver,” ATF regulations require the part to “provide housing for the hammer, bolt or breechblock, and firing mechanism.” [..] That means an AR-15 lower receiver is not a “firearm,” and the Government’s theory at trial was a non-starter.