On July 1, 1999, approximately 1445 mountain daylight time, a tailwheel equipped Piper PA-22, N3513A, registered to and being flown by a private pilot, was substantially damaged during the collapse of the left main landing gear on landing rollout at the Billings International Airport, Billings, Montana. The pilot and passenger were uninjured. Visual meteorological conditions existed, and no flight plan had been filed. The flight, which was personal, was to have been operated under 14CFR91, and originated from Helena, Montana, at 1234. The pilot reported that upon touchdown on runway 28R the aircraft began to drift left, and right rudder was applied to correct with no effect. As the drift continued, the pilot applied right brake, increasing brake pressure until he "was pushing against the seat." The aircraft continued to veer left, tightening its turn radius until it had rotated more than 180 degrees counter-clockwise. During the rotation, the aircraft settled to the left in line with the runway lights and came to a stop. The pilot and passenger exited and noted the left main landing gear separated from its bungee (shock strut,) but still attached to the airframe. Post-crash examination by an IA certified mechanic revealed that the left main landing gear shock strut tube had separated at its attach point to the left main wheel assembly (refer to DIAGRAM I). This would have allowed the release of the left main landing gear, permitting it to pivot into a flat (collapsed) position.