See what happens if you use a de-cap die on your Grendel brass first & then run them in your 6 ARC die with the expander rod/ball removed. If you still get crunched brass, It's probably caused by the die internal stop as mentioned by grayfox & Stonehendge. Your brass will need a little trimming before resizing if that's the case.
I've not reformed any fired Grendel brass to 6 ARC but I have reformed a bunch of new Grendel brass (Lapua, Hornady & Starline) to 6 ARC without any problems at all. The new Grendel brass might be a little shorter than fired brass... just a guess. I run the reformed brass through a separate expander die (.241" dia.) before final trimming.
I've not reformed any fired Grendel brass to 6 ARC but I have reformed a bunch of new Grendel brass (Lapua, Hornady & Starline) to 6 ARC without any problems at all. The new Grendel brass might be a little shorter than fired brass... just a guess. I run the reformed brass through a separate expander die (.241" dia.) before final trimming.
The trick is growing up without growing old. -- Casey Stengal