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... so after the 1st fire, you measure that distance, and annotate it. that's your fired shoulder size. after the 1st fire, that's where you want to resize the casing to. is this correct?
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And previously, by mehavey -
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- Get a cartridge headspace gauge (RCBS and/or Hornady). Then you measure the headspace dimension on the fired case(s) and adjust the sizing die's lock-ring to knock the shoulder back only ~0.002"(+/-). This case will now chamber/lock-up reliably and not overly stretch on firing -- doubling case life or more. But `ya gotta know what yer doin'.
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There's one school of thought which says a slight crush fit is fine. After all, a bolt-action has incredible camming power. Another school says to adjust the die for 0.001" to 0.002" smaller than the measured fired case length. That way, you don't have a round that won't chamber when you want it to on a hunting trip.
As long as you aren't unnecessarily resizing to SAAMI minimum, you aren't over-working the brass near the head. This means longer case life (more reloadings) and less risk of case head separations.
There are stories of some guy who didn't understand headspace and was experiencing case head separations every 15th round or so during a match. An experienced shooter checked his ammo and found it was undersized by something like 0.015" or more. Apparently, that is more stretch than brass can handle...
Another thing that can occur is actual failure to fire. I believe this is related to how well your extractor holds the case, but it is possible for the firing pin to drive the case forward until the shoulder touches and not fire the primer.
Excessive headspace is something to understand first, and then set up your equipment to avoid creating it. For a variety of reasons.