Heard about this on Iraqivet8888’s gun grips at least it’s some cover by law
26 U.S. Code § 5848 - Restrictive use of information
U.S. Code
Notes
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(a)General rule
No information or evidence obtained from an application, registration, or records required to be submitted or...
As long as the lower was bought just as a lower, it can be either. You can make a pistol into a rifle, and back into a pistol. You cannot make a rifle into a pistol.
"First, an AR pistol does not have a buttstock and is not intended to be fired from the shoulder like an SBR. Second, to build an AR pistol you must not use a rifle receiver that has been barreled and/or stocked as a rifle action, the first build of a lower will determine what the lower is from...