BATFE's headquarters' website has this pdf file explaining gun laws for the public.
On pages 73 and 75 (of the pdf) it shows shotguns with barrels shortened to "less than 16 inches", and states that "Since this weapon exhibits a barrel shorter than 16 inches, it is subject to the NFA regulations governing minimum dimensions..."
Uh-- the NFA limits for shotguns are 18 inches, not 16. The 16 inch limit only applies to rifles (because after WWII the government found to its embarassment that it had sold loads of M-1 carbines, as surplus, with 16 1/4 barrels). Shotguns with barrels under 18" have been subject to NFA registration since the statute was enacted in 1934.
Considering that possession of an unregistered shotgun with a barrel under the limit is a federal felony, with a prison sentence of up to ten years, you'd expect headquarters would be a little more accurate in its description of the law to the public....