I'm working through my memories, and there are a lot of things that come to mind, but chief among them was that along with other restrictions, banning unrifled barrels made garage gunsmiths from making illegal guns with steel pipe, for example. it required makers to spend more in manufacturing costs, in theory, making a cheap handgun harder to buy.
there were other things considered as well, and some were banned. guns with parts made of plastic, pot metal castings or sintered metals were considered "wrong" but not banned. Unrifled bores were illegal. It only costs a little bit to run a barrel through the machines, but that can only be done on expensive equipment.
The best way to describe this is that it was a "nuisance" law. It caused the larger, "legitimate" manufacturers such as Davis and jennings to spend money on what could almost be called useless or cosmetic features. It made it practically impossible for a mechanic at the gas station to make a gun that he could sell under the counter. It gave the government and police a single, simple object that they could identify with just the naked eye and prosecute clearly and easily.
We can compare this to a car. a firearm is illegal to own or sell with an unrifled barrel, and all you have to do is look down the barrel to have the answer. A car without headlights, seat belts, or other safety features is illegal to use on public highways in many places. Any police officer can see clearly if there aren't headlights, and the unsafe vehicle can be pulled over, examined, and eventually prosecuted for using an unsafe vehicle on public roads.
The owner of the jennings factory once said in an interview that if he could legally use unrifled barrels, he'd sell the equipment for scrap. With millions of barrels, the few cents that it cost to swage the rifling into the tubes was nothing but lost profits. He said that he did anything possible to cut costs and still produce a gun that could be safely fired.
