This is a 1/4-28 grip screw bottoming tap ideal for chasing the threads on anodized lowers to help get good conductivity between the Ghost Gunner and the lower receiver. This will help ensure you have a solid 5 Volt signal to the probe wire and help with more consistent probing on anodized lowers. Also consider using in conjunction with copper foil tape (sold separately) to further enhance the odds of good clean probing on anodized lowers.