AS THE PLT WAS LANDING AT A PRIVATE RESTRICTED LANDING AREA, THE ACFT TOUCHED DOWN LONG & CONTINUED OFF THE DEPARTURE END OF THE LANDING AREA. SUBSEQUENTLY, THE ACFT HIT A BUMP AS IT LEFT THE END OF THE RWY & THE NOSE GEAR WAS FOLDED BACK. THE PLT RPRTD THAT HE THOUGHT THE AVIALABLE RWY WAS MUCH LONGER. ALSO, HE STATED THAT THE BRAKING ACTION WAS ALMOST NON-EXISTENT ON THE WET GRASS & DOWNHILL SLOPE.