SBR is the dumbest portion of the NFA “law”.
Q: What is the purpose to a stock?
A: To improve accuracy.
Q: Is accuracy an important part of shooting?
A: It is a fundamental part of shooting.
Q: Does adding a stock to a pistol make it easier to hide?
A: Are you stupid or what.
Q: So what is the difference between a pistol and an SBR?
A: A pistol is more concealable.
Q: So why is an SBR more dangerous?
A: You got me!
There’s no real logic involved.
From what I can remember, the sawed-off shotgun was a weapon favored by bootleggers back in the 20’s because it seemed scary & didn’t require precise aim, so they banned it and somebody decided the same type of rule should apply to rifles as well — like the Thompson, which was also popular with gangsters of the day. The 16” barrel length is just an arbitrary thing some doofus came up with.