Are you trying to simply find the most accurate factory load posssibly available for your gun? Or are you trying to find the factory load that is most appropriate to be using for what you are doing, i.e. hunting white tails or elk?? If you are simply looking for a great load to punch paper, just start experimenting, I'm sure you will find a few different factory loads, probably in a few different weights, that your rifle likes somewhat similarly. At this point, if there is no decernable difference, cost of the ammo might be a consideration.(Then again, I do have a Browning 270 win that really only likes 130s as far as I can tell).
If you know you are going after a specific animal, that would probably give you a good jumping off point as to the different weights/types of bullets you probably want to start with, keeping in mind there is no such thing as too dead. Say if your 300 WBY really likes the 180 Barnes TTSXs the best but you are going after white tails, there really is no reason not to use that load. At this point, as long as you are not sacrificing any significant accuracy, choose the bullet best situated for your quarry that your rifle likes the best. That's what I do anyways.
Hope I didn't completely misunderstand the question and if I did, please disregard my ramblings!