Legal Question in Civil Rights Law in California

"Can they?" is not the right question. Of course they 'can'. They already did, didn't they?

Any item can be seized if it might be used as evidence in the case, or if it poses a threat to officers. A firearm in an assault case probably qualifies on both counts. If it really isn't being used as evidence, and if you are the lawful owner entitled to possession, you can petition the court for its release as part of the defense of the case. If you're serious about getting legal counsel to help you in this case, and if the case is in SoCal, feel free to contact me.