I agree with Oldtmr, only I will add when you remove the bolt, check the full inside of receiver for any place where grease might have built up or dirt or?? causing it to push the bolt up against the other side of receiver!

all bolts will get some wear and tear marks in that spot, but that seems a little more aggressive than it normally is,
but many of the bolts come in a chrome finish so many don't notice as much as when the Black coating wears as yours is!
when re installing things, maybe use dry lube over regular gun oil, if things maybe sit more than get shot , a good dry lube will still allow good lubrication , but won't a collect dust and dirt as fast! for you!