What brand mags are you running? I’ve had issues with the my Grendel similar to yours and I changed mags and solved that problem. Same with my X39. Only one certain mag will run flawlessly in it. My Grendel however just has a BCA barrel. Everything else is Anderson. The X39 is the same. BCA barrel and Anderson everything else. But there are lots of BCA rifles in our group and all seam to work just right.
