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Artful;n4002 wrote: If the cases had not been fire in the chamber before they would have to expand to fill the chamber – as the brass pulled back from the shoulder and expanded back towards the bolt the case would appear to shrink. That’s why I trim after first firing.
Another cause for shrinking cases when using squib or reduced loads (like your load of Red Dot) is that the pressure in the primer pocket is greater than the pressure in the chamber or case. This will force the case forward and shrink your case slightly. It is most apparent in your case’s base to datum line measurement or headspace. To preclude this, many shooters of light loads will drill out the flash hole in their segregated squib load cases to reduce the pressure in the primer pocket and thus preclude shrinkage.