The modular upper receiver block can be used in several ways for many different maintenance tasks. You may use just the lower half of the block by attaching it quickly to the upper receiver with the two provided receiver pins. This allows you to clamp the 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 receiver, 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. It is constructed of blue anodized aluminum.The modular upper receiver block can be used in several ways for many different maintenance tasks. You may use just the lower half of the block by attaching it quickly to the upper receiver with the two provided receiver pins. This allows you to clamp the 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 receiver 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 with blue anodized aluminum.