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 AR10 upper receiver with the two provided receiver pins. This allows you to clamp the AR10 upper receiver to a vise for maintenance work. Bolting the two halves of the upper receiver block fully supports the AR10 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 AR10 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.