Democrats proposed a law to ban people on the no-fly list from buying guns, and SCROTUS blocked it as a denial of due process.

The no-fly list is already punishment without due process. Being stopped from flying is more of a punishment than being stopped from buying a gun, and this is why the no-fly list must be abolished, or else redesigned so that people are only placed in it by a court's judgment.

At that point, it could also be applied to purchase of guns.