Real Avid Smart Fit, Vise Block, Gray Nylon, Designed for AR15 Lowers, Features Magazine Lock and Adjustment Handle AVAR15SFVB No more customizing static blocks" the revolutionary design of the Smart Fit(TM) Vise Block adjusts to fit every AR15 magazine well out there. The Smart Fit(TM) allows you to work on your lower receiver 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 your AR15. The included Pivot Lock holds the gun open giving you access to the chamber and barrel for easy cleaning.