For me it is temp sensitivity. I have hunted from 90f all the way down to -15f with the same rifle and load. I guess I look at what data shows the best accuracy for a given caliber and then weed out the temp issue. My last rifle purchase was a 6.5PRC, and unfortunately all things stacked up it is a slow shooter with the factory rounds I bought since no reloading brass was to be found. It is only 50FPS over the 6.5CM so I looked at what powder was best to get speeds back up to where they are supposed to shoot, but that situation is not the norm.