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In wearing out several 30 caliber barrels for both rimless bottleneck and belted bottleneck cases reloading thousands of rounds of ammo for them making all sorts of before and after case measurements, I'm convinced some popular beliefs about cartridge case behavior when fired is not well understood by folks.
First off, when the case is chambered in a rifle whose bolt has an in-line spring-loaded ejector, that ejector pushes the case as far into the chamber as its headspace allows. For rimless bottleneck cases, the case shoulder stops against the chamber shoulder and there's usually a small gap or clearance between the bolt face and the case head. For belted bottleneck cases, some will stop as their belt is pushed against the belt ridge in the chamber and there'll be a small gap between the bolt face and case head as well as between the case shoulder and chamber shoulder. In rifles with ejectors external to the bolt (Mauser styles that slide in a slot on the bolt), the case is a bit loose lengthwise in the chamber and will rest at various places depending on where the extractor pushes on it.
Second, when the firing pin strikes the primer some cases stay where they are in the chamber but others will move further forward. A belted bottleneck case with its belt held against the headsapce ridge in the chamber will stay there, the round will fire and the case shoulder gets blown forward while the case body grabs the chamber walls then the case head gets pushed back against the bolt face. If a belted bottleneck case has its shoulder against the chamber shoulder, the case shoulder may be set back a thousandth or so by firing pin impact increasing the space between the bolt face and case head, then the round fires and pressure expands the case body hard against the chamber walls while the case head gets pushed back against the bolt face. For rimless bottleneck cases, the firing pin drives then hard against the chamber shoulder and that sets the case shoulder back, the round fires and pressure expands the case body hard against the chamber walls while the case head gets pushed back against the bolt face. In virutally all instances, the case length from case head to case mouth is shorter after firing than before 'cause the case expands in diameter when its fired and that draws brass out of the chamber neck. Do your own before and after measurements with the right tools and you'll see this happen.
After a case is resized by a die, it gets longer from head to case mouth 'cause you've made its body and neck diameters smaller and that cartridge brass has to go somewhere. With the right type of sizing die and correct tools to measure what's happening, you can resize cases just enough to enable proper chambering for accuracy, safety and long case life.
The headspacing parts of case and chamber are established for safety. It's best if only a few thousandths inch difference between the case and chamber dimensions exist. Otherwise, accuracy will go down hill and the case may stretch too much causing metal fatigue such that the case cracks or splits letting hot, high pressure gas escape onto your body parts. Done correctly, managing head space is safer than crossing a street in traffic when you've got the walk signal.
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