The NcSTAR® VISM AR-15 Upper Receiver Block is a modular upper receiver block that can be used in several different ways for maintenance. The lower half of the block can quickly attach to the upper receiver with the 2 provided receiver pins and allows you to clamp the AR upper receiver to a vise for maintenance. What's more, bolting both halves to the upper receiver fully supports the upper receiver to prevent damage from torque when installing new parts. The upper half of this block inserts into the receiver like a bolt carrier group. The bottom half of this block can be bolted to the upper half with the provided steel bolts to fully support the inside and bottom of the receiver while leaving the receiver optics rail open for accessories. To top it off, the upper receiver block has a hole through the length of the block to act as a bore brush guide. Constructed with corrosion-resistant anodized aluminum.