Since I'm so full of good questions recently.......what do you look at to decide what powder your gonna load when your developing for a new rifle?
Start by learning a bit about powder burn rates in general and how that relates to the bore vs case volume. Study some burn rate charts (the Western Powders chart has the best visual representation IMO), but also keep in mind that those charts are an attempt to describe a three dimensional attribute in one dimension.
With some understanding of burn rates, look at load data for your cartridge, and focus on the top few powders for velocity. (In most cases the accuracy load, if listed, will be one of those powders too.) Comparing the burn rate of those powders to your charts will give you an indication that other powders within the same range may also work well. Consider density too though; sometimes a powder of the correct burn rate is too bulky, so it doesn't work simply because you can't get enough in the case.
That's how I start off. There are exceptions of course, like when building mild loads, working with cast bullets, or other things that stray from traditional published data. There are some rules of thumb to apply with those type of things, which we can discuss if you want.
