Arbitrary limits on minimum barrel length for everyday rifles and shotguns were imposed via the NFA on American gun owners due largely to media-created hysteria over crime. Such firearms are available over the counter with no additional restrictions in many Western countries with generally stricter gun control laws than ours. To purchase a short-barreled firearm or simply modify your own lawfully-possessed rifle or shotgun in the US requires registration of the firearm with the Federal government and payment of a tax.
There is no evidence such controls have had a positive effect on public safety or crime rates, without such evidence there is no rational basis for keeping such arbitrary restrictions in place. It is past time these restrictions were removed.