I don't know why there seems to be people confused on this issue about the differences.
There is but two categories A. Receivers that never have wore a stock being original bare frames or pistols
B. Receivers that have been stocked rifles or shotguns (long guns)
According to the 1930's Federal NFA law that applies in all states of the US, making a pistol from a rifle (category B) is illegal unless you have a paid and received a Federal tax stamp for such activity. What defines category B ? Any frame or pistol that has ever been stocked whether by the manufacturer, gun smith, owner, or anyone.
This is the main definition that the DOJ is upholding and it has never been challenged in court. Contrary to popular belief, the TC Supreme Court Case did not change this.
Again, the only thing that matters is if it has EVER had a stock installed. This issue alone is the determining factor when big brother decides to make an example of someone
and send them to prison. It will not matter that you registered it with your local city PD. It will not matter that it was bought as whatever. The one issue that will burn you is
has this receiver ever wore a stock and if it has why is there a rifle barrel less than 16 inches on there if in fact you do not have a Federal Tax Stamp. I think that is pretty clear.
I happen to talk to someone in law enforcement recently and was told that a number of people get caught every year with short barrel long guns.