Try a slower burning powder. 8208 is relatively fast for the Grendel. It's the most likely powder to short-stroke in the 18" midlength and 20" rifle-length systems that I've used. (The midlengths would likely function okay with 8208, but adjustable gas blocks had been turned down.)
Also, a heavier buffer would increase the likelihood of short-stroking.
A good gauge for tuning a system is to keep a few factory rounds onhand. I usually have a few AA 123gr Scenars and Hornday 123gr Amax factory rounds with me when doing initial trials. Besides tuning the system based on these rounds, it's been my experience that the barrel will show some indication of which bullet it likes best, and that's the first bullet that I focus my handloading efforts on.
Also, a heavier buffer would increase the likelihood of short-stroking.
A good gauge for tuning a system is to keep a few factory rounds onhand. I usually have a few AA 123gr Scenars and Hornday 123gr Amax factory rounds with me when doing initial trials. Besides tuning the system based on these rounds, it's been my experience that the barrel will show some indication of which bullet it likes best, and that's the first bullet that I focus my handloading efforts on.
Drifter