As the tundra tire equipped airplane slowed to 25 knots during the landing roll on a gravel strip, the left main landing gear leg separated at the wheel attachment bolt holes. The airplane ground looped, and both wingtips were damaged. Metallurgical examination revealed multiple fatigue cracks originating from the bolt holes. The airplane had been used extensively for off-airport landings, both on wheels and skis. The failed gear leg had been inspected for cracks 806 flight hours and 16 months prior to the accident.