I'm also really curious about casing design. How and why exact lengths, diameters, angles, etc. were chosen and what they do for the performance of the cartridge.
Hopefully I can find materials pursuant to this.
If you ever find them, please post the sources. This is a topic I have been looking at, spent hours going through reports on DTIC and there is very little information out there on case design.
The best concept presentation on cartridge design that I have found starts section 4-76, page 4-117 in AMCP 706-247 Ammunition Series:
This section at least provides basics on what a cartridge case is supposed to do, but specifics such as shoulder angles, have not found that yet.