On 1 February 1955, "Barbero" entered Mare Island Naval Shipyard for her a conversion under project SCB 118, equipping her to launch the Regulus I nuclear cruise missile. She was thus redesignated SSG-317 and recommissioned on 28 October 1955 under Lt. Cmdr. Samuel T. Bussey as the second submarine equipped with the Regulus missile, having been converted in 1953. In this role, "Barbero" was equipped with a hangar housing two missiles and a launcher on the after deck. One of the limitations of Regulus was that the firing submarine had to surface, the missile then being rolled out onto the launcher and fired. Regulus I also required guidance from submarines or other platforms after firing. She operated off the coast of California, firing her first test missile near San Clemente Island on 14 March 1956, until April 1956, when she transited the Panama Canal and joined the Atlantic Fleet, based at Norfolk, Virginia. From there, she served as an integral part in the nation's defense against nuclear war by carrying out early versions of the deterrent patrols that later became a regular feature of the submarine force after the introduction of the fleet ballistic missile submarine. She conducted patrols in the Atlantic for the next three years, under a policy of having one Regulus boat in each ocean.