In an AR build, one of the most critical elements of construction is to ensure that you don't stress elements of the upper receiver when installing the barrel. This makes the builder's choice of upper-receiver vise block critical. This Real Avid Lug-Lok Upper Vise Block is purpose-built to avoid inward pressure on the receiver during the application of torque. Each Real Avid Lug-Lok Upper Vise Block uses a special extension that locks into the barrel lugs inside the star chamber of your barrel extension. This transfers all the stress from the receiver to the barrel itself, which completely negates any chance of stress fractures to your upper receiver. The Real Avid Lug-L Mfg: Real Avid