Must have if using a suppressor - SureFire Bore Alignment Rod To Check Suppressor Alignment

This rod has saved me on a couple of build where the muzzle device was not perfectly square. If the rod is not in the center of the bore when checking then you either have an issue with the barrel or muzzle device, use a different sized shim to clock the muzzle device roughly 180 degrees from the original position then if the rod is still leaning the same direction then the barrel shoulder is bad but if the rod points to the same part of the muzzle device then there is an issue with it.