I have never had a belted cartridge but have read some info on it but it still confused me on how they get their headspacing.
Some say headspace off the belt & some say headspace just like a rimless bottleneck case??
So I cant really comment but it interests me on what you need to measure headspace & how a belted cartridge die set up differs?
I don't think this was explicitly answered above, so here it goes...
Headspace can be a "fluid" term, but essentially it is the distance between the bolt face and the case head, when the cartridge is stopped in the chamber. In other words, how much the case has room to move rearwards. On a belted mag, this can be
either the belt or the shoulder, whichever is shortest.
Factory ammo and new brass usually have the shoulder pretty far back, and thus the cartridge will stop against the belt, leaving as much as 0.010" between the case shoulder and the chamber shoulder. Upon firing, this will drive the case head backwards whatever the belt headspace allows, and will blow the case body and shoulder forward whatever the shoulder space allows.
Once it's fired, the case shoulder will likely now be very close to the chamber shoulder, and if you were to neck size only, the case would likely stop against the shoulder and not the belt, thus becoming a "shoulder headspace" cartridge like a rimless design.
The problem inherent to belted cartridges, and particularly when using the "standard die instructions", is that when you full size the case, you can push the shoulder all the way back to that factory new distance (or even further).
As described above, you're allowing the front part of the case to blow forward a long ways, and the rest to blow backwards. This stretches the case, thus thinning it. This usually occurs right in front of the belt. After a few firings, the wall thins excessively, and a head separation occurs.
If instead, you bump the shoulder back only a couple of thousandths from it's fired state, just like you'd do for a rimless/beltless cartridge, the case moves far less.
In summary, unless you need extreme chambering reliability to hunt dangerous game, and don't want to reuse your brass more than a couple times,
you actually measure case dimensions and set up a belted die exactly the same as you would a rimless/beltless cartridge. Measure base to shoulder on a fired case with a comparator, then adjust your die to push the shoulder down only 0.001" - 0.003", so there's enough space to chamber easily, but not enough for excessive stretching during firing.
