Worth every bit of money - OpticsPlanet Exclusive Midwest Industries OPMOD AR15 Upper Receiver Rod - Nickel

The receiver rod fits very nice and snug onto the upper receiver and barrel extension. I imagine this will also work as a proper gauge to make sure the barrel extension is milled and seated correctly (straight) on the barrel other wise it wouldn't fit into the receiver rod with the upper receiver on. The upper block on this receiver is nice since it provides extra lock up and helps disperse the force exerted on the upper receiver when torquing down the barrel nut.