Lug-Lok® Upper Vise Block (AR15)Advanced Thinking, AR15 Upper Block. 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. Thats 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