I have a Glock 17 that went to just under 60k rounds that required a new trigger spring. The funny part is, when I took it apart to clean it & replace the spring, I realized something was missing. The left rear slide rail on the frame was MIA. No idea when it came out, since I never really cleaned that gun (trainer) unless it became so filthy that the gritty sounds in it started to bother me. So who knows how long it was firing with a missing rail. So I sent it to Glock, they put it in a new frame, cleaned it up, replaced everything except the slide and barrel, and even installed new trijicon sights. They removed my Vickers mag release and installed a stock one, but my Vickers was in the box when the gun shipped back about a week or so after I sent it. The price was the shipping cost to Glock.
M4 bolts- it was policy in the unit to replace M4 bolts whenever a barrel was replaced, in the event the barrel coded out. M4 barrels lasted about 18 months in that unit. Every bolt failure I saw is believed to have been the result of de-tempering as a result of heat from excessive (cyclic) full auto fire from people being stupid. I didn't allow that kind of foolishness with my guys, and we never had a bolt head failure. Our M4s were FA, not burst.