The AR Upper Vise Block is designed for use with AR-style upper receivers when tightening barrel nuts to the proper specifications and attaching/removing a muzzle device. The block is made from solvent resistant black polymer. The clamp is compatible with A3 flat top receivers as well as some AR10 and AR308 receivers. For your safety, Trinity Force recommends all weapon repairs and modifications be performed by a qualified gunsmith/installer with an understanding of the weapon platform that the part is being installed on. Always make sure the firearm is unloaded and pointed in a safe direction when working.