THE NOSE GEAR WAS SHEARED OFF WHEN THE ACFT CONTACTED A DITCH DURING AN OVERRUN LANDING. UPON ARRIVING AT THE PVT AIRSTRIP THE PLT CIRCLED THE RWY AND DECIDED IT WAS SUITABLE FOR LANDING. TOUCHDOWN WAS MADE 400 FT DOWN THE RWY AND NORMAL DECELERATION WAS OBTAINED THROUGH THE USE OF BRAKES AND AERODYNAMIC BRAKING. SHORTLY THEREAFTER, THE ACFT DECELERATION SLOWED AND GROUND SPEED INCREASED. THE PLT VERIFIED THROTTLES AT IDLE AND ACFT CONFIGURATION. THE ACFT TRAVELED OFF THE END OF THE RWY, CONTACTED A POST AND A DITCH, CONTINUED OVER A TWO LANE HWY AND CAME TO REST IN A SECOND DITCH. PLT WAS ORIGINALLY INFORMED THE AIRSTRIP WAS 3000 FT LONG WHEN IN ACTUALITY IT IS 2250 FT LONG. THE GRASS WAS ALSO WET FROM MORNING DEW. AN ABORTED LANDING WAS NOT PERFORMED BECAUSE OF PWR LINES AT THE END OF THE RWY. A 5 KT TAILWIND WAS PRESENT.