Traditional clamp style upper blocks fit poorly and can exert enough inward pressure to damage the receiver when applying torque to a barrel nut or flash suppressor. That's bad design. So, Real Avid designed an upper block that leverages the strongest part of the gun, the barrel lugs inside the star chamber. The Lug-Lok is precision machined and precisely fits into all Mil-Spec uppers. It mates perfectly with the barrel lugs, while allowing you to rotate the upper to be able to work on different sides. There are 2 vise pins that allow you to hold the upper at multiple working angles and prevent unwanted rotation during high torque application. The lock knob keeps Lug-Lok seated into the barrel lugs. The result is the safest, easiest and smartest way to lock down an AR15 upper into a bench vise.