Thread: Case head separation imminent

Case head separation,what to look for and how to avoid it comes up fairly often on here so when prepping some of my "beater brass" today I found one and thought I would section it to show what to look for.


A line in the brass just above the case head web, often this is not perfectly symmetrical and can't usually be felt on the outside.
Feeling down inside the case with something like a paperclip bent 90degrees at the bottom you will generally feel the groove.


So what causes it and how best to avoid it?
First we need to understand what is happening when we pull the trigger.
The firing pin drives the case forward in the chamber until it is stopped by usually the shoulder of the case or the belt/rim contacting the corresponding part of the chamber.
This creates a space between the case head and the bolt head called "Headspace"
Now that the case can no longer move forward the primer is detonated and pressure in the case begins to build.
As pressure builds the thinnest part of the case at the neck expands to grip and seal off the chamber and release the projectile.
The rest of the case body expands and is held by the chamber but the rear of the case thickens near the case head till it can no longer stretch out to the chamber and as pressure continues to build to peak this part of the case is stretched rearward until stopped by the bolt.
This stretching happens in the same place every shot and no resizing method will put that brass back where it came from.
The greater the Headspace the greater the stretching with each shot.
When rezising that extra brass is squeezed in and up and the case gets longer....If you have to trim brass often you probably are over sizing the case.