Rudy Etchen's fired 4 million rounds though his 870

[Synchronizor]

I don't know for sure, but he most likely had to replace springs from time to time. Probably the extractor & firing pin at least once, as well. Action bars, shell latches, and/or the ejector may have needed replacing or repair at some point. If he stuck with the same barrel for all those shots, the bolt assembly may have needed an oversized locking block fitted to keep proper headspace; though that would depend on the loads he was using. If it was mostly lighter target loads, the headspace may still be fine.