By any chance are you using a magpul 10/20 round pinned mag? If so try a different mag.
I had the same problem with mine when I first shot it. I built it all up myself and figured it was something I put together wrong. I double and triple checked and everything was perfect. Turns out my rifle just doesn't like the 10/20 round magpul mag I originally bought. What was happening for me was the round wasn't feeding out of the mag correctly into the chamber and the bolt carrier would bite onto the top of the case and smash it into the front of the magwell at the end of almost every other cycle.
I've put about 1500 rounds through my rifle, all with 5 other magpul mags and I've yet to see this happen again.
Edit: added pictures
the top of the case that shows the dent similar to yours.
the bottom of the case that jams onto the magwell