The EVG Ergonomic Forearm Vertical Grip fits the Picatinny rail of your M4 or M16, giving you a compact vertical forward grip and allowing you to take a comfortable fighting stance. Polymer ergonomic grip with rubberized back for comfortable grip surface. Checkering on finger grooves side for added control. Capative thumb nut can't be lost. Storage compartment holds batteries or other items like firing pins. No tools required to install and remove. Lifetime warranty.