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I think it is two things. CoolBarrelBill hit on the primary reasons. Combine over sizing with hot (they don't even have to be max) loads, and I am surprised that your brass lasted 6 firings.
To get the longest life out of brass, you have to size them as little as possible in order to reduce the expansion (during firing) and compression (pushing the shoulder too far back), use moderate loads only, and anneal the necks (not the base or anywhere near the base). Those three things will help your brass last much longer.
If you use one of the gauges that you put on the datum line at about the midline of the shoulder, you can find out very closely about what the length of your chamber is. There is of course some expansion and springback during and after firing. But measuring at the datum line before and after sizing will tell you how much you are setting the shoulder back. I like to set the shoulder back about .002 thou for semi-autos. For a single bolt gun, I only neck size. If you are setting the shoulder back .009 thou, that could easily explain why your cases are separating.
Also make sure to trim brass to the proper length, and that case necks don't thicken too much. Either of those will cause the bullet to be held too tightly in the throat and raise pressure much more than you expect.
When you fire the round, the only place the brass can go after it has filled the chamber cavity is forward toward the throat/barrel. This (along with sizing) causes the case necks to get longer, and can also cause neck thickening. But usually, length is the issue.
To get the longest life out of brass, you have to size them as little as possible in order to reduce the expansion (during firing) and compression (pushing the shoulder too far back), use moderate loads only, and anneal the necks (not the base or anywhere near the base). Those three things will help your brass last much longer.
If you use one of the gauges that you put on the datum line at about the midline of the shoulder, you can find out very closely about what the length of your chamber is. There is of course some expansion and springback during and after firing. But measuring at the datum line before and after sizing will tell you how much you are setting the shoulder back. I like to set the shoulder back about .002 thou for semi-autos. For a single bolt gun, I only neck size. If you are setting the shoulder back .009 thou, that could easily explain why your cases are separating.
Also make sure to trim brass to the proper length, and that case necks don't thicken too much. Either of those will cause the bullet to be held too tightly in the throat and raise pressure much more than you expect.
When you fire the round, the only place the brass can go after it has filled the chamber cavity is forward toward the throat/barrel. This (along with sizing) causes the case necks to get longer, and can also cause neck thickening. But usually, length is the issue.
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