Here's what the official BATF regs say:
Short barreled rifle-a rifle having 1 or more barrels less than 16" in length and any weapon made from a rifle, whether by alteration, modification, or otherwise, if such weapon as modified has an overall lenght of less than 26 ".

Short barreled shotgun- A shotgun having one or more barrels less than 18" and any weapon..... less than 26" in overall length (paraphrased as it reads exactly as the short barreled rifle rules)