blackpowder is just barely chemistry, more like engineering.

carbon, sulphur, and potassium nitrate, in a particular ratio.

potassium nitrate is watched, and reported in large quantities, or particular form, but can be manufactured by most people that can follow a recipe.

regulating the propellant cant stop it from being made.

also someone really didnt think it through by regulating "receivers"

they regulated what is most often the easiest part to manufacture. the core parts [barrel, bolt, chamber] are difficult to build, require tech to build from stock, and are sold off the shelf, while receiver needs 4473 as if it was a fully functional firearm, and that is the part that can be built, from a 2x4 or a billet of material, depending how long you want it to last.