California Code, Penal Code - PEN § 833.5

(a) In addition to any other detention permitted by law, if a peace officer has reasonable cause to believe that a person has a firearm or other deadly weapon with him or her in violation of any provision of law relating to firearms or deadly weapons the peace officer may detain that person to determine whether a crime relating to firearms or deadly weapons has been committed.

For purposes of this section, “reasonable cause to detain” requires that the circumstances known or apparent to the officer must include specific and articulable facts causing him or her to suspect that some offense relating to firearms or deadly weapons has taken place or is occurring or is about to occur and that the person he or she intends to detain is involved in that offense. The circumstances must be such as would cause any reasonable peace officer in like position, drawing when appropriate on his or her training and experience, to suspect the same offense and the same involvement by the person in question.

(b) Incident to any detention permitted pursuant to subdivision (a), a peace officer may conduct a limited search of the person for firearms or weapons if the peace officer reasonably concludes that the person detained may be armed and presently dangerous to the peace officer or others. Any firearm or weapon seized pursuant to a valid detention or search pursuant to this section shall be admissible in evidence in any proceeding for any purpose permitted by law.

(c) This section shall not be construed to otherwise limit the authority of a peace officer to detain any person or to make an arrest based on reasonable cause.

(d) This section shall not be construed to permit a peace officer to conduct a detention or search of any person at the person's residence or place of business absent a search warrant or other reasonable cause to detain or search.

(e) If a firearm or weapon is seized pursuant to this section and the person from whom it was seized owned the firearm or weapon and is convicted of a violation of any offense relating to the possession of such firearm or weapon, the court shall order the firearm or weapon to be deemed a nuisance and disposed of in the manner provided by Sections 18000 and 18005.