1991greg,
You get all the parts EXCEPT for the one that is considered the gun itself. With an AR, the part that has the serial number on it is considered the gun and has to be purchased from an ffl. This part is the lower receiver on the AR15. The other stuff can be purchased anyplace because it is just machined parts. I link on the FN FAL, the part that is the gun is the upper receiver.
You can look at it this way. If you have a 20" metal tube, is it a gun? A bin full of parts isn't a firearm. Only the part that makes it all work is considered a firearm. I found it confusing that a RDIAS was considered a machine gun, yet it is just one little tiny part. You can put it in an AR15 and have a registered machine gun...but the machine gun is the RDIAS. The AR15 is just a bunch of parts.