The barrel diagnostic is a complete inspection. 

First we visually inspect the bore, chamber and crown with a top quality bore scope.  We then measure the bore diameter and the groove diameter to see that everything falls with in acceptable parameters.  Next we cast a short Cerro-safe slug in the barrel and slide it thru the barrel.  If there are any tight or loose spots they will be detected here.  We next pour a long Cerro-safe slug and slide it thru the barrel.  This will show problems a short slug will not, such as consistency of barrel twist or changes in the land/groove width .  Last we pour a chamber cast this gives us an exact picture of your chamber.  It shows how well the chamber was finished at the factory, the neck diameter and a great deal of information about the most critical part of your rifle, the throat. We measure its diameter, length, lead angle, concentricity and wear. 

All of this information gives us a clear picture regarding the rifles overall accuracy capabilities.  If things are not right with the barrel then all of the other work is just wasted time and money.