What you do with it makes it one or the other. The NFA is a farce that we have to work on getting rid of.When you buy a bare receiver through an FFL, it's listed, or classified as "other" on the paperwork, the part that the ATF classifies as a "receiver".
Here's something that confuses me, is how is an 80% receiver classified as a rifle or pistol? If you do the final machine work to turn it into a working firearm, you can build it either way. So what legally makes it a pistol, or a rifle?