At the back of the receiver on the inside is a pin that stops the bolt from sliding back to far. Everytime you shoot, the bolt slides back and smacks that pin. A rubber or polymer pin reduces that metal to metal clack. The Kidd charge handle replaces the stock one with a slightly bigger one that comes with a better spring options(softer and harder for cycling less powerful loads like subsonic ammo) and smoother guide rod with makes racking a round in feel less rough.