I used a DPMS LPK on my latest build. I've only got 100 rounds through this gun. I noticed the trigger had some grit and creep that wasn't there at first. I took the hammer and trigger out of the lower and noticed a groove worn into the trigger face. There is noticeable depth to this groove, almost like the sear is harder than the trigger face and created the groove upon resetting. The sear looked good. I've included a couple pictures of the trigger face. What's going on? Was the trigger not hardened?