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Headspace in a rimmed case is the same as for a rimless case - the distance from the stopping point of the case to the face of the breechblock.
The difference is that for a rimmed case, the stopping point is the face of the breech, where for a rimless case it is a point in the shoulder of the chamber.
In a rimmed case, headspace is some defined distance greater than the rim thickness. For example, the .30-30 rim thickness spec is .062"-063"; the headspace spec is .063"-.070".
Belted magnums really are rimmed cases. When H&H came up with the .375 H&H Magnum, they wanted to use a rimless case for easy feeding. But they found out that the tiny shoulder would not be enough for longitudinal case support against the firing pin. Without that support, misfires and inaccuracy (due to inconsistent primer ignition) would result.*
So they built in what amounts to a new rim, the belt. Contrary to common belief, the belt does not add strength to the case; it is too far back to support the case walls.
If the shoulder of a rimmed or belted case is set back during re-sizing, firing in a rifle with proper headspace will simply blow it back out to fill the chamber. Only if the rear of the case can move back far enough to exceed the elasticity of the case material, will the case separate. That will not happen if the headspace is within specified limits, since those limits take the elasticity of the case into consideration.
There is a belief that with a case like the .303 British or .30-40 that there is some need for the case shoulder to be tight against the chamber shoulder, and that setting the shoulder back too far in reloading will cause excess headspace. That is not true. The only advantage of keeping resizing to a minimum is greater case life, since there will be less flexing of the brass at the case shoulder. But that has nothing to do with headspace.
*A recurring problem with the .35 Remington!
Jim
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