As the federal government licenses gun dealers, manufactures and importers and regulates their business, they can prohibit the sale of firearms from dealers to civilians. They regulate explosives, so they control gun powder and primers. They can deem anything that travels in interstate commerce, like steel, cannot be used to make firearms. They pass a law that all existing firearms must be transferred through a FFL (no private person transfers).
If they had wanted to do that, dont you think they would have done it already? Under Clinton they cracked down on "kitchen table" dealers but did not outlaw them. Did anyone raise a ruckus about the 2A then? No.
Again, the Feds can pass any law they want. But then they gotta enforce it.