"Machine Gun: ref. NFA Title II, sec 5845 (b) Any barrel length and stock configuration is legal. $200.00 Transfer Tax
Rifle/Carbine Title I firearms: 16 inch minimum length barrel as measured from the end of the barrel or permanently attached accessory, to the bolt face and a minimum overall length (OAL) of 26 inches with the permanently attached stock extended. For a semi-auto UZI with a metal folding stock, that means the OAL is measured with the stock open. It can also have a permanently attached wood stock. If it has a quick-detach wood stock, it must meet the OAL requirement with the stock off. Note that some states are more restrictive then these federal regulations.
Short Barreled Rifle (SBR): ref. GCA of 1968 Title I, Chapter 44,921 (7), (8) Any rifle or Carbine having a barrel less than 16 inches long and an overall length less than 26 inches with the permanently attached stock, extended. $200.00 Transfer Tax.
Any Other Weapon (AOW): ref NFA, Title II, sec 5845 (e). Firearms with a rifled barrel length less than 16 inches or less than 18 inches shotgun, without a buttstock. This classification allows the use of a vertical forward grip but prohibits the installation of a buttstock. $5.00 Transfer Tax.
Pistol (Title I): Barrel length 16 inches or less without a buttstock. The pistol must have been originally assembled as a pistol and not a conversion of a rifle or carbine. A Pistol cannot have a vertical foregrip."
http://files.uzitalk.com/reference/pages/FAQ.htm#Barrel and stock