On January 17, 2003, at 1430 Pacific standard time, an experimental ME 262 reproduction, N262AZ, sustained substantial damage following a gear collapse during the landing rollout at Snohomish County Airport (Paine Field), Everett, Washington. The airline transport pilot-in-command was not injured. The airplane is owned by a private party and was being operated as a visual flight rules (VFR) post production test flight under Title 14, CFR Part 91, when the accident occurred. Visual meteorological conditions prevailed, and a flight plan had been filed for the local test flight that originated from Paine Field approximately 30 minutes prior to the accident. During a telephone conversation with the Investigator-In-Charge (IIC), the pilot reported that he experienced a problem with the airplane's landing gear and utilized the emergency gear extension system (nitrogen blow-down) to lower the gear. The pilot stated that after activating the system, the landing gear extended and the green gear down indicator lights illuminated. After successfully lowering the gear, the pilot initiated a landing at the departure airport. He reported that shortly after touchdown, the left main landing gear collapsed and the aircraft veered to the left, eventually exiting the runway edge and traveling down an embankment that's adjacent to the runway. Post accident inspection of the aircraft's left main gear hydraulic system revealed that the hydraulic actuator locking slide was undersized which allowed the gear to collapse (unlock) when placed under a load. See attached report for additional information. During the accident sequence, the airplane sustained substantial damage to the fuselage.