HAO's 416D castle nut is the classic HK416D style castle nut  /receiver extension nut
 
 
Made in Taiwan
BB type: 6mm 0.2g BBs
Power source: 11.1V PTW, AEG / 9KG green gas GBBR
Power: 90 m/s = 0.81J with 0.2g BBs
J / cm2:  2.835 J/cm2
Design and manufacture for Airsoft use. No live ammunition using function.