Two bits of info straight from the Form 1:
Definition (1)(c):
Firearm. The term “firearm” means: (1) a shotgun having a barrelor barrels of less than 18 inches in length; (2) a weapon made from a shotgun if such weapon as modified has an overall length of less than 26 inches or a barrel or barrels of less than 18 inches in length; (3) a rifle having a barrel or barrels of less than 16 inches in length; (4) a weapon made from a rifle if such weapon as modified has an overall length of less than 26 inches or a barrel or barrels of less than 16 inches in length; (5) any other weapon, as defined in 18 U.S.C. 5845(e); (6) a machinegun; (7) a muffler or a silencer for any firearm whether or not such firearm is included within this definition; and (8) a destructive device.
Instruction (2)(j):
Description of Firearm and Markings. (1) Item 4a. If you are modifying an existing firearm, enter the name and location of the original manufacturer. If you are creating the firearm, enter your name, city and state. (2) Item 4b. The types of NFAfirearms are listed in instruction1c. (3) Item 4c. Specify one caliber or gauge. If there is another designation, indicate the designation in item 4h. (4) Item 4d. Show the model designation (if known). (5) Items 4e and 4f. Specify one barrel length and overall length in items 4e and 4f as applicable. Note: if the firearm has a folding or collapsible stock, the overall measurement is to be made with the stock extended. (6) Item 4g. Do not alter or modify the serial number of an existing firearm. Enter the existing serial number in item 4g. to register the firearm. (7) Markings: The maker is required to mark the firearm with his name, city and state. All markings are to be in compliance with 27 CFR 478.92 and 479.102.