You guys are going to question this one, but it is true, and I am a Glock armorer.
The test, with your Glock unloaded of course, checked 3 times and then in the preloaded condition, that means you've cycled the slide and it is ready to shoot, but with an empty chamber, point the muzzle straight up, and press the trigger but not enough to fire, if you fire, cycle again, notice the movement of the slide, it will slide to the rear when the trigger is pressed, if it moves more than 1/8" replace the recoil spring assembly, or if you want to experiment, get a wolffe spring, cut the keeper off the plastic guide rod and go. I have successfully removed the keeper at the end of the guide rod, but you have to work slowly and a hair drier will help get the plastic soft, use a dental pick to remove it. The only advantage of the keeper is that it makes it a captured spring and is nice for disassembly and reassembly.
Now why?? A long time ago I'm pretty sure J.D. Jones of Magnaport fame, was hunting, and was shooting nearly straight up and his Glock no go bang. The glock fire control system is balanced between the recoil spring and the trigger spring versus the firing pin spring. With a worn recoil spring and pointed straight up, the act of pulling the trigger, pulled the slide back enough to engage the safety plunger and block the firing pin. Basically pulling the pistol out of battery. When level or pointing down, the weight of the slide gives the extra help.
Even with this condition present, if your shooting as normal, level or down, the pistol will probably be 100%.
Try this test on your Glock and see what happens. How many of you bite this time??
