Army's answer above is not entirely correct.
He states that state law pre-empts the landlord. The landlord's lease language is enforceable as between the two of you. Meaning that if the landlord discovers you possess a handgun he CAN if he wishes, enforce the lease terms and kick you out for violation of the lease or force you to dispossess yourself of the handgun.
As between you and the state of California. Army's answer is correct. Nothing in the lease affects your legal/criminal rights. The lease provision does not makt it a a criminal violation for you to possess the handgun in your apartment (provided it would be legal otherwise). Anything that you could legally do if the lease provision didn't exist you can still legally do. The lease agreement doesn't change anything, other than the fact that the landlord can kick you out if he learns of your handgun possession.