I have a phone and a pen
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Most of it is horseshit. "Ghost guns" are simply any firearm made in a home workshop. As long as the gun is never sold, it does not have to have a serial number. If it leaves the possession of the maker, it is subject to serialization through multiple federal laws. What he's doing is a sop to the idiots.
The AR-15 pistol bit is interesting. Personally, I think the things have a very narrow purpose and most gun owners don't care if he does subject them to the NFA. I do think that should be Congress, not Executive Order.