Painter, when you asked how a light recoil spring could cause that break, I questioned my own understanding of what was happening when the slide cycled, so I pulled out my 75, cleaned off the back face of that lower part of the slide, smeared some red grease on the front side of the block in the receiver, cycled the slide, and the red grease transferred from the front side of that block to the back side of that piece on the slide. Then, to make sure the grease wasn't on too thick, decreasing the distance between those two parts and producing a false result, I repeated the test with a very thin smear of lipstick and with the same result - lipstick transfer. Those two surfaces make contact when the slide cycles. When you install one of those little polymer recoil buffers, doesn't it serve as a buffer between those two contact points?
As far as being surprised that this sort of break doesn't happen more often, the recoil spring spreads that recoil energy out over time, and your body sucks all of that energy up during the slide stroke, then, so as long as the recoil spring is of appropriate strength, by the time those two surfaces impact, there's not all that much energy left, and the impact should be relatively light, and it may even fall short of impact entirely. On the other hand, if the recoil spring is too light... SLAM!
I am not a gun smith, and all of this is based off of observation and interpretation. If I am completely wrong, please someone let me know. But for now, the grease and lipstick transfers between those two surfaces seem like compelling evidence.
