Those definitions tend to be inconsistent and strange though. They often concern themselves with things like pistol grips vs thumbhole stocks, which only impact the ergonomics and the appearance of a firearm, not the function.
And even a barrel size limit is a strange thing to regulate. Short barreled rifles are not inherently more dangerous than regular size rifles. The only reason they are regulated today is as a holdover from a piece of legislation that would have banned handguns.