I think BATFE has tried to scare people away from trying to convert AR-15s to full auto by claiming that possession of even one part for an FA gun is illegal even if that one part could not, by itself, make the conversion.
I would be interested in hearing about any real case in which someone was arrested, charged, and prosecuted for simply owning, say, an auto bolt carrier or an FA hammer, whether or not they also owned a semi-auto AR-15.
While BATFE has declared some single parts to be machineguns, like the bolt carrier of the AK-47), AFAIK that has not been done with any AR-16 parts. (Please don't tell me about the shoestring! I mean actual parts.)