In California law enforcement can take your firearms if you are detained on a 5150 hold.  Getting them back may be difficult, but not impossible.  W&I Code § 8103(f)(1) makes a person ineligible from owning, possessing, controlling, receiving, or purchasing a firearm (or attempting to do so) for five years unless you take advantage of the statutory provisions for a subsequent court order to regain your right to possess a firearm (W&I Code § 8103(f)(4)-(7).  If a motion is filed with the Court a hearing will be set.  Having an attorney is not mandatory, but it is very difficult to prevail without one.

At such a hearing, hearsay, in the form of declarations, police reports, criminal history information; and any other material and relevant evidence (subject to exclusion pursuant to E.C. § 352; i.e., where the “probative value” of such evidence is outweighed by its “prejudicial effect”) is admissible. (Subd. (f)(5) & (9).) At this hearing the standard of proof is preponderance of the evidence and the District Attorney has the burden to establish your gun rights should not be restored. Although  an individual’s right to possess firearms is of fundamental constitutional stature (i.e., the Second Amendment), it remains subject to the state’s traditional authority to regulate firearm use by individuals who have a mental illness.

Under subdivision (f)(6), the People of the State of California, represented by the District Attorney, has the burden of showing at such a hearing by a “preponderance of the evidence” that the person petitioning to regain his or her right to own, possess, control, receive, or purchase a firearm “would not be likely to use firearms in a safe and lawful manner.”

I hope this article has been helpful.  As a local Torrance Criminal Defense Attorney, if I can be of assistance let me know.