I'm not disagreeing or anything, but would like to hear some insight into how specific brass manufacturer can shoot better or worse than others.
Far as brass goes, in order to determine if your gun likes one better than the other, the only way to really do it is to eliminate every other variable. Case length, charge in terms of powder and weight, bullet, primer and OAL would all need to be exactly the same from one brass maker to the next to be able to find any statistical variability in accuracy attributed solely to case manufacturer. Temperature, wind drift and elevation could even be included as variables if you really wanted to get crazy. And there may be others I'm not thinking of.
That would be a difficult task, but not impossible