Freaked out now - Case head seperation.

[junkdude]

Belted cases headspace using the belt. The headspace gauges are short and only involve the recess reamed for the belt. The chamber could be half an inch too long and the gauges will not detect it. Or, the chamber could be short and once again the gauges will not show it.
A belted magnum reamer cuts both body and belt recess, in a single operation.
If the headspace is within limits, the case will fireform to the chamber. This expansion may or may not be excessive. If the case is fully FL sized, with the shell holder contacting the die, the case body will be sized as far as possible. Perhaps way more than necessary. This can contribute to separations.
In addition, the thickness of the belts can vary. If belts are on the thin side, or if the rim recess is maximum, and cases are sized to the limit, the possibility of separations in increased.
Resizing the case as if rimless avoids complications.

Tiraiq has nailed it with good advice

This is the main problem with a belted cartridges, unfortunately a bad design. Two dimensions have to be correct, belt to throat and belt to head for both the case and the gun.

Cartridge can be thought of as nothing more than a gasket. Anchoring the gasket in the wrong place or with out of tolerance dimensions and applying pressure will cause it to stretch and possibly fail.

 

[bearhunter]

Tiraiq has nailed it with good advice

This is the main problem with a belted cartridges, unfortunately a bad design. Two dimensions have to be correct, belt to throat and belt to head for both the case and the gun.

Cartridge can be thought of as nothing more than a gasket. Anchoring the gasket in the wrong place or with out of tolerance dimensions and applying pressure will cause it to stretch and possibly fail.

The process is the same, IF THE REAMER IS GROUND TO PROPER DIMENSIONS.

The only difference is the measuring point.

Once the case is fireformed it's all moot, other than if the reloader insists on full length resizing and pushing the shoulder back to as new spec.