On September 16, 2002, about 1500 central daylight time, a homebuilt RV6, N440CM, registered to a private individual, experienced collapse of both main landing gears during an aborted takeoff from the Bay Minette Municipal Airport, Bay Minette, Alabama. Visual meteorological conditions prevailed at the time and no flight plan was filed for the 14 CFR Part 91 personal flight. The airplane was substantially damaged and the private-rated pilot and one passenger were not injured. The flight was originating at the time of the accident. The pilot stated that during a previous flight, the engine rpm decreased 150 to 175 rpm after switching fuel tanks. He elected to land at the Bay Minette Municipal Airport. While there, 5 gallons of 100 low lead fuel were added and he talked with a mechanic at the airport. He also performed an engine run-up and the engine sounded, "...OK. Not rough as before." He decided to depart intending to climb to 3,000 or 4,000 feet, remaining close to the airport, before deciding to continue to the initial intended destination airport. The engine ran OK until just after liftoff, where the engine began to run rough again. He aborted the takeoff, and landed uneventfully but was unable to stop the airplane on the runway. He intentionally ground looped the airplane to avoid a fence and a 8-foot embankment. The pilot stated to the Federal Aviation Administration inspector-in-charge (FAA-IIC), that after landing at Bay Minette, he ran the engine which ran rough. Maintenance personnel at Bay Minette advised him that he had a bad right magneto, and repairs could be performed. He (pilot) again ran the engine reporting no magneto drop during the run. He elected to takeoff and continue to his original destination airport. Following recovery of the airplane, in the presence of the FAA-IIC, the engine was started but could only be operated to between 1,000 and 1,200 rpm due to damage to the engine mount. During the engine run, a magneto check was performed but a rpm decrease could not be detected. Examination of the right magneto revealed that the points were not opening resulting in an inoperative magneto.