Guys, vamo has it exactly right.
The BASIS of the restriction was concealability, as hso said, but it only made a lick of "sense" in the context that handguns were to absolutely be restricted out of common availability, PRIMARILY. That was really the core of the matter. The "SBR" and "SBS" stuff was simply created because the powers that were knew that folks wanting a concealable weapon would just make one from a hunting rifle or shotgun if they couldn't get their hands on a pistol or revolver.
So, they make "firearms made from a" rifle or shotgun expressly just as [strike]illegal[/strike] heavily regulated and taxed as handguns were to be.
But then, amazingly and oh, so thankfully, handguns were stricken from the bill before it was passed.
The shotgun and rifle corollary was simply left in as a bit of sloppy housekeeping.
But then as the years went by, the idea of a sawed-off shotgun (especially) grew in the public consciousness. Since it was so very especially illegal, it MUST be a devastating, extra lethal weapon!
Once you grasp that these laws are just a matter of legislative sloppyness, and that the law tying silencers into the whole mess comes from largely socially irrelevant anti-poaching concerns, the whole stinking pile that is the NFA starts to be seen as more and more of an embarrassment and outrage.