"Atlanta"s career had not yet ended, however. She underwent an extensive modification at the San Francisco Naval Shipyard. Reinstated on the Navy list as IX-304 on 15 May 1964, the vessel was converted to a target ship for studies of the effects of high energy air explosions on naval ships. The changes included cutting her hull down to the main deck level and erecting various experimental superstructures, designed for guided missile frigates and guided missile destroyers, on her deck. In these configurations she was subjected to explosions to determine whether or not the experimental structures could satisfactorily combine essential lightness with equally essential strength and blast resistance. These three tests were conducted off the coast of Kahoolawe, Hawaii, in early 1965, known as Operation Sailor Hat. "Atlanta" was damaged, but not sunk, by the experiments. She was laid up at Stockton, California, sometime late in 1965. Her name was again struck from the Register on 1 April 1970, and she was sunk during an explosive test off San Clemente Island on 1 October 1970.