Case Shoulder
The angled transition from the case body to the neck that serves as the headspacing datum on a rimless bottleneck cartridge.
The case shoulder is the tapered cone where the wider body of a bottleneck case steps down to the narrower neck. On a rimless cartridge this angled surface is the part that stops forward travel in the chamber, so the shoulder, not a rim, controls headspace. The exact shoulder angle is part of the cartridge specification and varies from gentle slopes to the steep forty-degree shoulders favored on some improved designs.
For handloaders the shoulder is where precision lives. Each firing pushes the shoulder forward against the chamber, and sizing must move it back a controlled amount, a step called shoulder bump, usually a couple of thousandths of an inch. Bumping too little causes hard bolt closure, while bumping too much shortens case life and can open up dangerous excess headspace.