http://www.recguns.com/Sources/IIF1.html
-snip
"A short barreled shotgun is any shotgun
(shoulder fired, smooth bore) with a barrel of less than 18" or
an overall length of less than 26", or any weapon made from a
shotgun falling into the same length parameters. A short
barreled rifle is a rifle (shoulder fired, rifled bore) with a
barrel length of less than 16", or an overall length of less
than 26", or any weapon made from a rifle falling into the same
length parameters (like a pistol made from a rifle). In
measuring barrel length you do it from the closed breech to the
muzzle, see 27 CFR sec. 179.11. To measure overall length do so
along, "the distance between the extreme ends of the weapon
measured along a line parallel to the center line of the bore."
27 CFR sec. 179.11. On a folding stock weapon you measure with
the stock extended, provided the stock is not readily detachable,
and the weapon is meant to be fired from the shoulder. A
destructive device (DD) is a explosive, incendiary or poison gas
weapon, or any firearm with a bore over 1/2", with exceptions for
sporting shotguns, among other things (see discussion below).
Any other weapons (AOW's) are a number of things; smooth bore
pistols, any pistol with more than one grip,(but see below)
gadget type guns (cane gun, pen gun) and shoulder fired weapons
with both rifled and smooth bore barrels between 12" and 18",
that must be manually reloaded (see discussion below). These
definitions are simplified, to see if a specific gun is a title 1
or 2 firearm one needs to refer to the specific definition under
the statute(s), and possibly consult with the Technology Branch
of ATF. There is also case law on the issue of whether a
specific item falls into one of these categories."-snip
