AS a further note, and regarding the example Brian found for us, "belted cases" such as those shown, will in many if not most cases, fire by "head spacing" on the belt.
However, Many chambers are slightly over size - manufacturing toleriences again - and if that is so, the brass will move ahead during firing until stopped by the belt or shoulder and then the case head will be slammed back to the bolt face as discribed in my last post.
For almost all belted chambers, the brass should be sized to head space on the shoulder just as with non-belted brass, rather then relying on the belt for proper head space.
For and in most current firearms, the belt serves more of a, "cool factor" then a useful function.
Some folk even have a thing about any "belted" cartridge, but I have used them for years and use them for what they are, good and useful hunting cartridges. Besides, there is that "cool factor."
Crusty Deary Ol'Coot