Re: 30-06 brass
I would suggest that brand/volume case capacity variations are pretty far down on the list of priorities for load development.
It may appear possible to anticipate a need for greater case volume, but this is by no means assured.
For reasons of economy and availablity, I typically use Winchester (or Remington, but Winchester is usually the more available off the shelf where I buy), and because my regimen always includes the same phases of case prep, regardless of source, there is less value to me in seeking out higher quality/price brass.
In my load developement, I accept my case capacity as a given, then choose powder burn rate speeds based on pressure testing with various powders and the chosen bullet. Often one will find that individual case capacity varition within a brand lot are as great or more than variations between brands. I cull the really wildly incosistent individual cases, reserving them for foulers, etc. BTW, I measure with powder, not water, using a drop tube.
When pressure indicators get high and velocity increase slows down or goes away, i'm usually at or past max. When that's also happening with 10% or more excess space in the case, it's a clear indicator that my chosen powder burn rate speed is faster than optimal.
I stop pressure testing at the first signs of bolt drag on opening, or if two or more of the other pressure indicators suggest peaky pressures.
I have concluded that high presures do not gain as much in performance as they cost in bore longevity; and do my load development downward from an arbitrary and conservative max load.
On the other hand, if I cannot get enough powder into the case to generate moderately high working pressure signs and/or velocities are significantly below what published load data indicates a bullet/case combination is capable of, this generally indicates my chosen powder burn rate speed is slower than optimal.
Generally, a good load will be at or above 90% case capacity (with bullet seated, i.e., shaking the case will still move the powder around).
This percentage is sometimes referred to as load density percentage. Ideally, it will 100%, and and a small degree of compression is OK as long as you're not really well into the max pressure region.
By reducing powder charge mobility within the case, ignition consistency is improved, and this is one of the keys to developing a more accurate load.
Greg