Actually building a gun yourself is completely legal, and building an AR from an 80% lower is not that difficult with the proper tools and some knowledge. But doing so in a couple of hours is a exageration, it would take a bit longer than that.
But here is the question, if you can build a gun from an 80% lower you could also do so from a block of aluminum for an AR or block of steel fora 1911 or,any other firearms. How are they going to stop people from building guns from a block of metal.