@Freeman15,
Just because I am sick of the uneducated BS, here is the excert from the NFA Handbook. This Handbook defines the various firearms, and what it takes for them to be considered a NFA firearm. Here is the part about shotguns...
CHAPTER 2. WHAT ARE “FIREARMS” UNDER THE
NFA?
Section 2.1 Types of NFA firearms
The NFA defines the specific types of firearms subject to the provisions of the Act.
These definitions describe the function, design, configuration and/or dimensions that
weapons must have to be NFA firearms. In addition to describing the weapon, some
definitions (machinegun, rifle, shotgun, any other weapon) state that the firearm
described also includes a weapon that can be readily restored to fire. A firearm that can
be readily restored to fire is a firearm that in its present condition is incapable of
expelling a projectile by the action of an explosive (or, in the case of a machinegun, will
not in its present condition shoot automatically) but which can be restored to a functional
condition by the replacement of missing or defective component parts. Please be aware
that case law is not specific but courts have held that the “readily restorable” test is
satisfied where a firearm can be made capable of renewed automatic operation, even if it
requires some degree of skill and the use of tools and parts.
2.1.1 Shotgun. A shotgun is a firearm designed to be fired from the shoulder and
designed to use the energy of the explosive in a fixed shotgun shell to fire through a
smooth bore either a number of projectiles or a single projectile for each pull of the trigger.
A shotgun subject to the NFA has a barrel or barrels of less than 18 inches in
length.
The length of the vast majority of shotgun barrels is measured from the muzzle of the
barrel to the face of the breech on a line parallel to the axis of the bore. In the case of a shotgun having a chamber(s) not an integral part of the barrel, such as a revolving
shotgun, the barrel length is measured from the muzzle to the front of the separate
cylinder and does not include the chamber. NOTE: Any muzzle attachment such as a
compensator, choke device, muzzle break, etc., is not included in the barrel length
measurement unless the attachment is permanently affixed to the barrel. Acceptable
methods for permanently attaching a device to a shotgun barrel are deep penetrating, full fusion, gas or electric steel seam welds or high temperature silver solder.
2.1.2 Weapon made from a shotgun. A weapon made from a shotgun is a shotgun type weapon that has an overall length of less than 26 inches or a barrel or barrels of less than 18 inches in length.
The overall length of a firearm is the distance between the muzzle of the barrel and the
rearmost portion of the weapon measured on a line parallel to the axis of the bore.
If you noticed, the words "sawed off" do not appear, because they do not care about it. It is a slang term, and not a technical description therefore it has nothing to do with the law.