It was Nosler & Hornady, my only other choices are S&B (not available at the time) and Starline (not sure if they were available); I think Remington was still making 6.8 SPC brass at the time but I wasn't going to buy from them. After the problem was addressed I still had about 300 cases that seem to be holding up well so I will probably trash the barrel before I run out of brass.No, but when you use different parts from different manufacturers, it's a good place to start looking for the cause of the problem. You issue above is worse than belted mags that I have seen. I would look at different brass.
I checked headspace when I assembled the upper but I wasn't checking case growth at the time; I used the method the die manufacturer recommended for setting up your sizer die at the time.