Run hard enough and long enough, just about every part in an AR can break.
Good quality bolts typically break somewhere in the range of 60-100K rounds.
Upper receivers wear out around 300k-500k rounds.
Ejector spring are cheap. I think Blue replaces his once a year.
Personally had a magazine catch break on my Bushmaster around the 20 year mark. Of course, I had an extra on hand, so it was a 5 minute fix to be up and running again.
