You're referring to what's called "shoulder clearance." It happens with new belted cases when they're all the way into the chamber and their belt's against its shoulder in the chamber. When such a case is fired, it's shoulder gets pushed forward against the chamber shoulder and its head gets pushed back against the bolt face. That leaves a small gap between the belt and its chamber shoulder. This also happens with rimmed bottleneck cases such as the .30-30 WCF. As long as shoulder clearance is within specs, it's OK.
Lots of folks use an RCBS Mic to measure fired belted case shoulder headspace. That Mic is set up to do that for belted cases. Setting those cases' shoulder back 1 to 2 thousandths typically lets them headspace on their shoulder instead on their belt. Best accuracy happens doing this. I've been doing it for decades.
Last edited by Bart B.; November 28, 2012 at 04:21 PM.