I have an early push feed Ruger M77 and I don't like the snap over only feature. I don't know where to get a replacement, maybe you know a source for tang safety extractors? Unless you have worn barrels out on your push feed M77, then you are not going to see failure mechanisms that high volume shooters experience. I can go to the local Gunstore and I doubt I will find one used bolt gun where the barrel was shot out. I will find some where the barrel was rusted out, but I think the average bolt gun is fired less than 100 rounds.
Given time all extractors will failure. I have been on the firing line when push feed M70 extractors failed, and the match director had a kit with replacement extractors! I have seen M700 extractors fail, I have not worn out an AR15 extractor but it is a matter of time. I have had a Garand extractor break and disappear. Extractors are thin but highly stressed. This is another reason to lubricate cartridges for gas guns as rounds between extractor failure will increase. One of the many good reasons POF has fluted its E squared chamber.
In the book, Random Shots page 65, American test FN rifles were breaking their extractors sooner than Belgium test FN rifles. It turns out the Belgium rounds were coated with a light oil which broke the friction between case and chamber, reducing extraction forces . Once the American's got the oil removed, Belgium extractors broke at the same rate as American extractors! Hatcherites would consider this progress.
I would prefer not to stress any of my claw extractors un necessarily. When the pre 64's went out of production, owners who had rifles with worn out extractors were in a pickle. I have one pre 64 with a modified M1917 extractor. Gunsmiths could mill and machine a M1917 extractor to fit, but it was not cheap!. The alternative was a single shot M70, one you manually pushed the round in the chamber, and knocked out with a cleaning rod. I think I took that extractor off and put on a newly made M70 claw, extractors which I have stockpiled for the day they again become hard to find.