This is an excellent tool. It is both an upper receiver lapping tool as well as a "reaction rod". Purchasing these tools separately would be added cost. The X-Caliber tool is well machined, fits well into the barrel / receiver. When used as a lapping tool it is excellent with great bore alignment and tight tolerances (Aero M5E1). I have this tool for both Aero M5 receivers and barrels as well as standard AR15. High quality and if you can catch them during one of their great site-wide sales, they're a real get.