Quote Originally Posted by jmoore View Post
"Fatigue" engineering really didn't get off the ground until the post WWII era. When they designed this action there were only basic notions of the whole design concept.
Perhaps you are thinking of fatique, aluminum and the DeHavilland Comet? de Havilland Comet - Wikipedia, the free encyclopedia

Not being in school at the time I am not certain pre WWII design practices. However, I am looking at my 1941 edition of Modern Metallurgy for Engineers, on page 152 is a very nice S-N curve for carbon steels, and it is dated 1921. On page 153 is the relation of enruance limit to tensile strength and brinell hardness, and that is dated 1929.

At least for carbon steels, fatique lifetime was well defined and understood.

I don't know the design practices of Britishicon Small arms designers. They might have designed the bolt for a certain number of rounds in a 20 year lifetime and added a bit of margin. I would have designed the bolt for an infinite loading cycle, but that is me.