This replacement part is displayed as number 285 on KM4 series KM4 Series/CQR Series/KM16SR12 diagrams and functions to better secure the inner barrel.
This replacement part is displayed as part number 400 on RM4A1/CQB/T6/QRF/T10 diagram. Screw for the end sling plate on the ERG, AEG2.5/3, AND series guns....