The lawyers representing the convicted assassin said in new court documents that a bullet was switched in evidence at trial and new forensic tests show Sirhan is innocent.
They said sophisticated audio tests recently conducted on recordings from the assassination night show 13 shots from multiple guns were fired - five more than Sirhan could have fired from his small pistol.
They are calling for Sirhan to be released from prison or granted a new trial. The state attorney general's office had no comment.
Sirhan was convicted of killing Kennedy at the Ambassador Hotel. He's serving a life sentence.
Sirhan, now 67, was denied parole after a hearing last March where he denied any memory of shooting Kennedy on June 5, 1968, moments after he claimed victory in the California presidential primary.
The Associated Press contributed to this report.