Pistol braces could be 100% outlawed... I don't see what the actual problem is.
Pistol brace use could be punishable by death. I don't understand why one person would be in danger.
The whole prohibition on short-barreled rifles and shotguns is an anachronism, makes no sense, and should be abolished.
First, as originally written, the National Firearms Act of 1934 would have also applied to pistols and revolvers. That's the whole reason the law places a minimum length on rifles and shotguns—namely, it was intended to close a loophole and prevent people from simply shortening rifles and shotguns into handguns. But the bill didn't get enough votes to pass Congress with the handgun stuff in place, so handguns were stripped out of the NFA. With no extra regulation on handguns, the whole reason for the ban on shortened rifles and shotguns evaporated.
Second, without the same NFA regulations also being applied to handguns, the regulations of rifles and shotguns makes no sense. For example, say I cut down a single-shot .22 rifle without going through the process to legally create an SBR. That's a federal felony. But there's no prohibition whatsoever on owning, say, a Glock 17, which carries 17 rounds of a vastly more powerful cartridge. Likewise, I'm a felon if I cut down a .410 double-barrel shotgun, but I can legally buy a Taurus Judge revolver that caries five rounds of .410. A short-barrel rifle in .223 requires a tax stamp and BATFE approval. A .50 AE Desert Eagle that fires bullets with twice the energy does not. The distinction between handguns and SBRs is purely arbitrary.
Third, the whole notion that handguns, along with other guns similar in size to handguns, were undeserving of any sort of legal protection has now been twice rejected by SCOTUS. Indeed, while the Supreme Court has declined thus far to say exactly what sorts of arms are or are not protected by the Second Amendment, there is one class of guns that it has said are protected and we know are constitutionally protected:
handguns. Outlawing other types of guns because they are "too small" and similar to handguns makes no sense in a world where the Supreme Court has explicitly said that handgun ownership in the home is a constitutional right.
Fourth, the prohibition on short-barreled rifles and shotguns is based on outdated societal norms. Throughout most of history, the American public frowned on the
concealed carry of firearms. It was thought that someone going about with a concealed handgun or similar firearm had "something to hide," and if they were honest and upright they would be carrying the gun openly, which was legal pretty much everywhere. Indeed, the whole notion of regulating handguns and other small firearms though the NFA was to crack own on
concealable guns. Today, however, concealed carry is not only legal in every state,* but societal norms have reversed such that it is now open carry that is usually greeted with suspicion and alarm. Banning guns because they are concealable makes no sense in a world where concealed carry is not only legal, but expected.
* There is at least in theory a de jure legal process to obtain a concealed-carry permit in every state, although some states like Hawaii and New Jersey pretty much make it functionally impossible to ever actually get one.