Part of the puzzle is that we attempted to stick with bolt action rifles too long, seeing that they didn't work that well in ww1 was a wake up call, if I recall my college history class correctly.
Either you are thinking of the Pederson device, or how well machine guns did in trench warfare compared to rifle fire. I have read something to the effect that 300 machine guns stopped the July 1916 British Somme offensive. In a large part responsible for 60,000 causalities in one day!
But what does surprise me, after reading a book on the German experience on the Somme, was the lack of German progress on a semi automatic rifle. I have read in this book numerous personal accounts of Germans shooting their M98 rifles till the barrels glowed. While this strains belief, I do believe that it shows their infantry men had long periods of high rate sustained rifle fire in defense of their positions. But the Germans stuck with a five round rifle all the way up to WWII. They could have at least put a 10 round box magazine on the thing.