As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
FIFTEEN MINUTES INTO FLIGHT PILOT HEARD LOUD BANG, NOTED GEAR DOWN AND LOCKED. UNABLE TO RETRACT GEAR. AFTER AN UNEVENTFUL LANDING, AIRCRAFT TURNED OVER TO MAINTENANCE. INSP FOUND NO HYDRAULIC LEAKS. GEAR WOULD NOT RETRACT. FOUND LEFT GEAR ACTUATOR'S PISTON BROKEN OFF INSIDE ACTUATOR ALLOWING FLUID TO CIRCULATE WITH NO PRESSURE. AIRCRAFT USED FOR FLIGHT TRAINING. GEAR CYCLED OFTEN.