The is one of the best AR-15 upper receiver vise block tools you can buy made from hard anodized aluminum gunsmiths love the QD modular design. This AR-15 vism upper receiver vise block is a vital tool and this part enables you to securely clamp your upper receiver into a bench vise without fear of crushing, twisting or damaging it. The AR15 upper receiver block can be used in several ways for many different maintenance tasks. You can use the lower half of the vise block by attaching it quickly to the upper receiver with the two provided receiver pins. This allows you to clamp your AR upper receiver to a vise for maintenance work. Bolting the two halves of the upper receiver block fully supports the upper receiver to prevent damage from torque twisting forces when installing or removing barrels from the receiver. The upper half of the block inserts into the receiver just like a bolt carrier group. The bottom half of the block can be bolted together with the upper half with the provided steel bolts. This fully supports the inside and the bottom of the Mil-spec receiver Models and it also leaves the receiver optics rail open for mounting accessories and optics. Unlike other upper receiver blocks that bolt/clamp to the exterior of the AR receiver. The upper receiver block also has a hole through the length of the block to act as a bore brush guide for cleaning out the bore of the barrel. Constructed from blue hard anodized aluminum for years of use.