Smart-Fit® AR15 Vise BlockThe Smart Fit AR15 Vise Block is the last lower vise block youll 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. This block can also be used with an inverted lower receiver to work on pistol grips or allow easy access to the underside of your AR15. For hassle