Although my rifle is not mentioned, CZ ZKM 617 auto loader, when I 1st got it, it would not shoot 3 shots in a row, and the cases were ballooned on the case head, visibly. After reading the manual, the warranty shop happened to be in town, and one that I knew well. I called the owner and asked if he had seen this and his reply was YEAH, We are the shop that is supposed to figure out what is wrong. You can send it now, and wait for 2-3 months, or just check back with me, and when we get it figured out, you can bring it in and get it back in a week, so that is what I did. After I got it back, it did work better, but not 100%, some ballooning still on the case heads, but the form that I got back said, " do not return the rifle until 300 rounds have been shot through it " I have not shot 300 rounds yet. Also only use Hornady ammo.
I suspect the head ballooning is going on with other semi's. Not enough pressure on the case to complete the seal long enough to support it from the bolt. The CZ has a very heavy bolt. Similar to the 10/22 but twice the length and weight. It may be a improper balancing act, spring pressure and bolt weight, versus the pressure of the cartridge. My idea, if it were not covered under warranty, was to thread the spring guide rod, and install a nut to adjust the spring tension, and possibly try different spring rates, lighten the bolt, but thread the holes so if that was the wrong direction, I could put set screws back in, to bring the bolt weight back to original.