[Bart B.]

F. Guffey,

As virtually all rimless bottleneck cases headspacing on their shoulder are only a few thousandths inch (less than .005") shorter in their case head to shoulder reference than the chamber length from bolt face to chamber shoulder reference, and the firing pin protrudes from the bolt face as much as .050" to .065" in its full forward position, the case shoulder gets driven hard into the chamber shoulder from its impact on the primer. The case is driven hard enough into the chamber that its head to shoulder length is shortened .001" or more when the round fires. It's the case shoulder stopping against the chamber shoulder that allows the firing pin to dent the primer enough to fire the primer. And the case head's a few thousandths away from the bolt face when this happens. On new belted cases, they stop against the chamber belt stop to hold them solidly in place when they're fired, but the case head is still a few thousandths off the bolt face. And rimmed cases have the front of their rim stopping against the chamber to stop their forward movement while the firing pin dents the primer; their case head's a few thousandths away from the breech face, too.

Rarely, if ever, does the extractor hold the case rim or belt back far enough to keep the case shoulder from touching the chamber shoulder when the round's fired. There's enough clearance from the bolt face to the extractor lip to prevent this from happening.