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Why do my .308 Lapua cases shrink in length after firing?

Please can you help me?

I trim my cases to .2005, within half a thou.

After firing the cases will be .2001 within half a thou.

Many thanks in advance.

Cam
 
Because the entire case - body, shoulder and neck - grow larger in diameter upon being fired. COAL lessens as the brass expands, why resizing is necessary before re-use.

Depending on your chamber's dimensions the amount could be a little or a lot. In your chamber it sounds like it's generous (factory build?) so this is normal.
 
I used to experience the same (shrinking case lengths) shooting new Lapua 308 brass in my factory Remington 700 action. With my new Kelbly Panda F-Class action, which appears to be a tighter fit with tighter tolerances, my cases actually grow in length.
 
As mentioned, your chamber is larger than your brass was before being fired. In a tight-neck, tight everywhere else chamber, your brass will not likely shrink much if at all. I have Lapua cases fired over 30 times in an HBR gun.
 

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