Unless otherwise stated, all excerpts of California Penal Code are as obtained on October 29th, 2007 from the CA Attorney General's Website. The only changes made have been reformatting for display on the wiki, and the removal of non-shotgun-related material.
Illegal Shotgun Configurations
Most of the questions asked about shotguns in California, are specifically related to whether a specific weapon is legal or not. Any shotgun which is federally illegal (full auto, etc) is also illegal in California. The state also has its own definition of short barrelled shotgun, which is illegal, as well as assault shotguns, which are also illegal.
Observations on California Shotgun Law
This is the "Reader's Digest" version of the legalese reproduced down below.
Shotguns on the assault weapons list are prohibited. This eliminates a total of four "cool looking" shotguns that were big during 1980s / 1990s action movies. You can tell how the legislature formed their opinions of what to ban.
Revolving cylinders are prohibited on all shotguns. Whether it's pump action or semi-automatic, the existence of a cylinder on a shotgun renders it an assault weapon. Just like the listed shotguns, action movies are responsible for this ban.
Pump shotguns are pretty safe. Without a revolving cylinder, there's no way for it to become an assault weapon due to having pistol grips, vertical handgrips, etc.
Pump shotguns can have pistol grips in California. This is re-stated from above, simply due to the overwhelming number of times folks ask this question.
Semi-auto shotguns are allowed one "evil feature". This is unlike the rifle and handgun sections of the penal code, and probably reflects the legislature making compromises with hunters within California. This is why the Beretta M3, which a semi-auto shotgun with a pistol grip, is legal for sale in California but the version with a collapsible (i.e. telescoping) stock is not.
Semi-auto shotguns may never have detachable magazines. This is in contrast to the rifle statute, which allows detachable magazines if there is no pistol grip, folding stock, etc. However, a Saiga-12 imported by an off-list manufacturer and equipped with a bullet button or other legally compliant magazine lock is legal.
If a shotgun's barrel is less than 18 inches, it's illegal. This is one of the most frequently asked questions about shotguns.
If a shotgun is less than 26 inches in overall length, it's 'short barreled', even if its barrel is 18" or longer. This is a strange point, but it's in the penal code.
Definition of Short Barrelled Shotgun
CPC12020, Article 2: (c)(1) As used in this section, a "short-barreled shotgun" means any of the following:
(A) A firearm which is designed or redesigned to fire a fixed shotgun shell and having a barrel or barrels of less than 18 inches in length.
(B) A firearm which has an overall length of less than 26 inches and which 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 which may be readily restored to fire a fixed shotgun shell which, when so restored, is a device defined in subparagraphs (A) to (C) inclusive.
(E) Any part, or combination of parts, designed and intended to convert a device into a device defined in subparagraphs (A) to (C) inclusive, or any combination of parts from which a device defined in subparagraphs (A) to (C), inclusive, can be readily assembled if those parts are in the possession or under the control of the same person."
Assault Weapon List, Shotgun Section
12276. As used in this chapter, "assault weapon" shall mean the following designated semiautomatic firearms: Rifle/Pistol sections deleted.
(c) All of the following specified shotguns:
(1) Franchi SPAS 12 and LAW 12.
(2) Striker 12.
(3) The Streetsweeper type S/S Inc. SS/12.
Assault Weapon Definition, Shotgun Section
12276.1. (a) Notwithstanding Section 12276, "assault weapon" shall also mean any of the following: Rifle/Pistol sections deleted.
(6) A semiautomatic shotgun that has both of the following:
(A) A folding or telescoping stock.
(B) A pistol grip that protrudes conspicuously beneath the action of the weapon, thumbhole stock, or vertical handgrip.
(7) A semiautomatic shotgun that has the ability to accept a detachable magazine.
(8) Any shotgun with a revolving cylinder.
-- SeanNewton - 29 Oct 2007