As seen on reddit, a layman’s explanation.
In 1934, rich people decided that they didn’t like poor people sneaking onto their land to hunt game, which was happening a lot because there was this economic thing called the “Great Depression” and poor people were “starving”.
So they called up a bunch of their pet congresscritters, and ordered a custom made law called the National Firearms Act of 1934.
This placed a $200 dollar tax on rifles with a barrel shorter than 16″ or an overall length less than 26″, shotguns with a barrel shorter than 18″ or an overall length less than 26″, and firearm sound suppressors of any kind. This ensured that those nasty poor people couldn’t afford guns which they might smuggle onto rich people’s land to hunt rich people’s deer, or suppressors that might prevent them from getting caught in the act.
Nowdays, $200 isn’t a whole lot of money, but the law is being exploited in a new way to restrict access. You see, this tax gets you a “tax stamp”, without which possession of the firearm is a felony. But the law doesn’t put any upper limit on the wait before the paperwork gets approved. So these papers are processed by one elderly guy named Frank, whose eyesight isn’t so good any more, and a typical application takes six months to two years to come back.
Naturally, modern folks who own guns aren’t too keen on this, but modern folks who hate guns have their pet congresscritters who are dead set against repealing any law that makes firearms ownership inconvenient, even if it makes no sense at all. Also, there is a whole three-letter agency devoted to enforcing this thing, and they don’t want to lose their jobs, so they have plenty of interesting and highly inventive arguments about how this is critical to the future of the nation, and how everyone died multiple times every week before 1934 because we didn’t have this.
So people looked into ways to do normal things while in compliance with the letter of the law.
So, the thing that looks like a stock on that thing that looks like a rifle is technically an arm brace, which makes that thing technically a pistol, which makes the National Firearms Act technically not apply to it. Now, you could say that this is a loophole which complies with the letter of the law while totally ignoring its spirit, but you could also say that the law has no spirit, and is nothing but a big technicality designed to f*** with people from the very beginning.
But however we got here, the fact remains that the thing in the photo is perfectly legal as it is, but with a slightly different stock it’ll get you a decade in the federal penitentiary.
This is why gun owners get kind of shirty about this kind of thing.
This is the same reason why some of the gun community – including the NRA – will never support repeal of the NFA, GCA and the Hughes Amendment.
Too many people have too much money invested in Class 3 weapons. Suppose you spend $25,000 on a Class 3 weapon, or several of them, and someone comes along and tells you that your investment will tank in the near future and your guns will be worth no more than a used gun of that make and model within a single day.
Would you support it, looking at it only from the perspective of money? The better question is what, other than a true commitment to the RKBA under God, would make you support it?