The Smart Fit Vise Block adjusts to fit every AR15 magazine well out there. The Smart Fit allows for work on lower receivers with a solid foundation for changing out buffer tubes or fixed stock receiver extensions. 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 the AR15. The included Pivot Lock holds the gun open giving access to the chamber and barrel for easy cleaning.