Not necessarily. At the cartridge’s ignition, a light load won’t always fully expand the case within the chamber. It does, however, develop enough pressure to force the primer out of the pocket and against the bolt face, at least as much as headspace will allow. An instant after the primer has been forced back out of the pocket, the pressure will begin to drive the case backwards towards the bolt face. As the case moves back, it reseats the primer, often flattening it in the process. When the case is extracted and examined, the primer will appear to have been used in an extremely high pressure load. In some instances, primers can be a good indication of pressure. In others, they can be extremely misleading and utterly worthless as pressure indicators.
