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.

Black powder guns, at least ones of antique design (but modern production), are federally ~unregulated already anyways. A 6 year old in North Dakota could order one mailed right now to his house, no background checks, right off the internet -- legally.

There is also the "felon carry" as its called late 19th century black powder percussion pistols, you can also order off the internet, regardless of criminal history and with no scrutiny of the chain of custody.

Making a bullet is definitely more difficult than printing a plastic gun handle (you need the bullet itself, and the cartridge fit it perfectly), and you have a non-zero chance to lose some parts of the body if you make a mistake.

Sigh. None of the ammunition used in modern (i.e. made in the past 125 years) uses black powder propellant.