Tool marks in the chamber, probably from stuck cases.
And that is what finally killed my old, beat up 742. Actually, I never observed tool marks; I just had cases that would stick in the chamber, and jams with the rounds that would come out of the chamber. I checked into having it repaired; Remington won't do it, and there aren't many shops that will. And it would have been expensive. So I bought a new Remington bolt action, instead.