The SI Multi-Function End Plate and Anti-Rotation Castle Nut are designed to update and upgrade the often neglected castle nut and endplate system. While the traditional endplate and castle nut works extremely well in retaining your receiver extension, the peening of the castle nut is not conducive to rebuilding. With the vast amount of aftermarket support for the AR system today, we felt that there’s a need for a secure system as adaptive as you are. Enter the MFEP and ACN. The MFEP and ACN offer an anti-rotate QD socket or sling loops built in. The Anti-Rotation Castle Nut is specifically designed to work with this end plate. Instead of peening the endplate to secure the castle nut, a set screw fulfills this function to allow for multiple uses as you upgrade, or update your build.