Sawed-off shotguns are illegal in some states (California), so it depends.
If you check on the Cal DOJ website the shotgun barrel/barrels can be shortened to not less than 18 inches and/or overall gun length of 26 inches.
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Now HERE is a very relevant post, on 3-D CENSORSHIP: It is worth noting that both Disqus and Fakebook CENSOR any mention of a website, www.CodeIsFreeSpeech.com that provides FREE downloads of 3D-printable firearm blueprints. And, since Facebook/Disqus logins are so prevalent for posting commentary across the Internet, they are very effectively suppressing this “Forbidden Knowledge” almost EVERYWHERE. Please go to the website, download some or all 10 of the free blueprints, and then, repost it to your friends and across the Internet, to the extent you can. Strike a BLOW against Fakebook and judicial CENSORSHIP! Support TWO amendments at once, the First and the Second!
California's shotgun laws are the same as Federal law: Minimimum barrel length of 18" and minimum overall length of 26".
You will note in the article that the author cut down the barrel of his shotgun to 19", still a legal length.
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.
Sawed off shotguns are not illegal in California.
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.