PEN § 17180

As used in this part, “short-barreled shotgun” means any of the following:

(a) A firearm that is designed or redesigned to fire a fixed shotgun shell and has a barrel or barrels of less than 18 inches in length.

(b) A firearm that has an overall length of less than 26 inches and that is designed or redesigned to fire a fixed shotgun shell.

(c) Any weapon made from a shotgun (whether by alteration, modification, or otherwise) if that weapon, as modified, has an overall length of less than 26 inches or a barrel or barrels of less than 18 inches in length.

(d) Any device that may be readily restored to fire a fixed shotgun shell which, when so restored, is a device defined in subdivisions (a) to (c), inclusive.

(e) Any part, or combination of parts, designed and intended to convert a device into a device defined in subdivisions (a) to (c), inclusive, or any combination of parts from which a device defined in subdivisions (a) to (c), inclusive, can be readily assembled if those parts are in the possession or under the control of the same person.