The Smart-Fit AR15 Vise Block is the last lower vise block you'll ever need. The patent pending block expands to fit any .223 / 5.56mm AR15 magwell, making loose, sloppy fitting blocks a thing of the past. Insert the block into the magazine well and simply rotate the adjustment handle to tighten the fit.Specifications and Features: Adjustment handle rotates to expand block for precise fit Pivot lock holds gun open to optimal cleaning position Secure fit improves accuracy during lower receiver builds Versatile design accepts inverted lower receiver for pistol grip installation