http://www.titleii.com/BardwellOLD/nfa_faq.txt
A short barreled rifle is a rifle (which is defined as a shoulder
fired, rifled bore firearm) 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.
(emphasis added)
