During his 2nd solo flight in the Robinson R22, the pilot performed 2 low 'G' maneuvers. The 1st maneuver was essentially the same as when he practiced it during a check-out flight. However, the 2nd maneuver produced a rapid right roll to approximately 60 degrees of bank. The pilot recovered in a descending right turn. After recovery, the aircraft seemed to be operating normally. Before landing, he practiced an autorotative approach to a power-on recovery without touching down. Several landings were made, but he stated all touchdowns were normal. A post-flight inspection revealed the main rotor spindles had bumped the mast at the teetering stops, resulting in dents up to .045 to .50 inch in the mast. Also, the main rotor spindle arms (horns) had contacted the droop stops, shearing the droop stop retainer bolt and bending the spindle horns. According to the manufacturer, the droop stop/bolt damage could only occur during a hard landing or negative 'G' flight. The flight manual and Safety Notice #11 warn: recovery from roll due to low 'G' must begin with aft cyclic first to prevent mast bumping.