From Calguns Foundation Wiki
Silencers are illegal in CA.
As used in Chapter 9 (commencing with Section 33410) of Division 10 of Title 4, "silencer" means any device or attachment of any kind designed, used, or intended for use in silencing, diminishing, or muffling the report of a firearm. The term "silencer" also includes any combination of parts, designed or redesigned, and intended for use in assembling a silencer or fabricating a silencer and any part intended only for use in assembly or fabrication of a silencer.
Any person, firm, or corporation who within this state possesses a silencer is guilty of a felony and upon conviction thereof shall be punished by imprisonment in the state prison or by a fine not to exceed ten thousand dollars ($10,000), or by both that fine and imprisonment.