The ATFE has gone on record as saying they don't consider magazines not in the rifle as constructive possession. The documentation is out there if you don't believe me.What if you owned a rifle with 9 foreign parts(sans mag) and had foreign mags laying around, but not seated in the receiver? It follows the same principle of owning an automatic trigger group for a "semi auto" rifle. It establishes intent.
Im sure they wouldnt kick down your door for having too many foreign parts, but this whole thing is just intriguing to me.