IF there is a little slop in your chamber, to the extent that the shoulder moves forward upon firing EVEN IF THE HEADSPACE IS CORRECT (ie., bolt-face to cartridge belt cut, which is the headspace measurement on a belted case!!) it will cause the case to stretch and an incipient separation to begin. How many reloadings afterward it will stand before the separation is complete depends on the hardness of the bras, etc. etc. I have a 7 Mag. that has this problem. Headspace is correct, BUT the shoulder moves forward because the bolt-face to shoulder dimension of the chamber is sloppy! So I never set the shoulder back when sizing, but the cartridges invariably have a thin spot just ahead of the web due to the first shot! So they are already a little weak.

Many a belted magnum's chamber was cut a little too deep, but the cut for the belt is still within specs. It would not hurt for you to have the headspace checked, it is a simple test and won't cost much. I have seen vbery few elted magnums in which the bolt-face to belt cut edge was too great, but it is of course not impossible. Please do get it checked, and let us know if you really have excessive headspace or not! Then at least you'll know if the barrel needs to be set back and the chamber recut.

( A little comment on headspace. If headspace is actually excessive, quite often the cartridge will enter the chamber too far for the firing pin to hit the primer hard enough to fire it. This is especially true of push-feed actions! If your rifle is giving a good indentatioon to the primer, I suspect there's NO HEADSPACE PROBLEM. But have a gunsmith check it anyway, to remove all speculation.)