Just because iam new and researching, how does one measure case volume correctly? This would make since to me as iam researching powders and there are several that have been used with success but after putting the top four in a shopping "cart" I thought well that's an expensive experimen a d went back to reading.
If you want to determine case volume in a meaningful way, it is not trivial. It's not so much that's it's "hard" or "difficult", but it requires a good, accurate balance, a little practice, and a lot of patience:
http://forum.accurateshooter.com/threads/case-volume-determination-pic-heavy.3896148/
FWIW - QuickLoad reloading software will provide a value for case fill ratio if you have entered the bullet dimensions, COAL, and a charge weight, but it may not even be close to the true value unless you also input an accurate case volume. In my hands, the preset case volumes aren't even close; you must measure the water volume of the specific brand/Lot of brass you're using, fired in your rifle. For example, the preset value for .223 Rem case volume is something like 28.8 gr. Lapua brass typically runs at about 30.8 to 31.1 gr. This is a difference of ~7.5% and can have a huge effect on pressure/velocity readouts. Case volume can also vary quite a bit by brand. It's a good idea to directly measure a new Lot of brass after it has been fire-formed.
As far as selecting a powder, the OP provided a list of possible criteria, which were generally all possible valid inputs into the decision-making process. However, the relative order of importance will likely depend heavily on the intended purpose. I would loosely categorize the criteria I use to select a powder for F-Class (F-TR) loading as follows:
1) Powder Burn rate
2) Temperature stability
3) Case fill/kernel size
4) Velocity
In my hands, the burn rate is typically a guide to a subset or "range" powders that will possibly work in a given cartridge for a certain bullet weight class. By virtue of the fact that I don't consider anything but single-base extruded powders, temperature stability will typically (but not necessarily for every single powder) be in an acceptable range. The case fill/kernel size are both indicators relate to "bulk density" and how much powder you can reliably get in the case without grossly compressing the load. Finally, the velocity for a given charge weight will give you some idea of how much energy the powder contains.
Notice that velocity is at the very
bottom of my list, as it is often possible to obtain sufficient velocity to hit a particular accuracy node/barrel time for a given length barrel and bullet weight with several powders within a specific
burn rate region (i.e. criteria #1 on my list). For example, Varget, H4895, IMR4064, IMR4166, VVN140, and Re15 (to name a few) can all fall within a fairly narrow burn rate region and can all be used in a .223 Rem or .308 Win with medium to heavy weight bullets. So you can usually narrow the list of potential powders substantially right away by considering only those in an appropriate burn rate range. As you apply the other criteria, the list will get shorter and shorter, hopefully leaving only one or two, possibly three likely candidates to go out and actually test for precision as Alex Wheeler described earlier in this thread.
The real key for powder selection as others have noted is that you need to have a very good idea of what you're trying achieve, and the factors that are most critical to reaching that goal. I reload for F-TR, meaning heavy bullets in .308 Win and .223 Rem cases, so burn rate and being able to get enough powder in the case are critical. We also shoot long (20+) strings of fire in F-Class, and temperatures can typically range anywhere from the upper 30s to the upper 90s, so temperature-resistant powders are a must in my opinion. Regardless of the specific discipline, with a little deliberation, a good list of the most critical powder selection factors can be made, and it can help in making an optimal powder choice.