Receiver blanks that do not meet the definition of a "firearm" are not subject to regulation under the Gun Control Act (GCA). ATF has long held that items such as receiver blanks, "castings" or "machined bodies" in which the fire-control cavity area is completely solid and un-machined have not reached the "stage of manufacture" which would result in the classification of a firearm according to the GCA.
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The following three photos are provided as examples. The first receiver has a solid, un-machined fire-control cavity area with no holes or dimples for the selector, trigger, or hammer pins. It does not meet the GCA definition of a firearm. The second receiver, shown from the top, likewise has a solid, un-machined fire-control cavity area. It does not meet the GCA definition of a firearm. The third receiver has a partially machined fire-control cavity and does meet the GCA definition of a firearm.