On September 3, 2019, about 1000 mountain daylight time, a Jackson/Marini Velocity XL-RG, N2XF, was substantially damaged when it was involved in an accident at Mack Mesa Airport (10CO), Mack, Colorado. The commercial pilot was not injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 business flight. The airplane departed Grand Junction Regional Airport (GJT), Grand Junction, Colorado, at 0925, and was en route to Ravalli County Airport (6S5), Hamilton, Montana. According to the pilot's written statement, the airplane was climbing to its assigned cruising altitude of flight level (FL) 180 (18,000 ft). Climb power was set at 2,700 revolutions per minute (RPM) and 30 inches of manifold pressure (MAP) that resulted in a fuel flow of 34 gallons per hour (GPH). As the airplane passed 15,000 ft, the propeller started to overspeed to about 3,450 RPM. The pilot reduced engine RPM and MAP and advised Denver air route traffic control center (ARTCC). He was given vectors to 10CO. The pilot made a power-off approach to runway 07 and waited until the landing was assured before lowering the landing gear. Because of excessive speed, the airplane touched down halfway down the runway. The nose gear was down but the main landing gear was not completely extended, negating the pilot's application of brakes. During the landing roll, the airplane slid off the departure end of the runway, struck an irrigation pipe that knocked off the left rudder, and came rest in a clover field. The airplane was trucked back to the pilot's hangar at Orlando Apopka Airport (X04), Apopka, Florida, for repairs. Bench testing and disassembly of the MT propeller and governor by Air Prop Specialists, Inc., in Mariana, Florida, revealed no discrepancies. The screws that secured the timing ring to the three brackets that were bolted to the crankshaft at the propeller stud flange had failed, damaging the timing disk and sensors. The pilot said that the damage occurred just prior to the overspeed and was not the result of the overspeed or landing rollout. He said the overspeed was caused by the propeller governor's reaction to the slight slow down of engine speed that occurred ten seconds prior to the overspeed when the timing ring attachment screws backed out, damaging the electronic ignition timing sensors. The interference to rotation and damage caused by the loose screws caused the slowdown from about 2,700 rpm to about 2,550 rpm. The propeller governor decreased propeller pitch to bring the propeller rotation back up to speed but once the obstruction of the ingested screws cleared and the friction was released, the low pitch blades allowed the propeller to overspeed.