A little back ground then my problem I have a Glock 20SF I've been shooting approx a month, mostly with a Lone Wolf 40 S&W barrel. I'm mostly shooting very lite loads with Berry's 135 grain clads and 6.0 grains of W231 in Star Line brass. I have very close to 600 of this particular round though it. I'm have to use a 12 pound recoil spring to get good case ejections, which is very light and the slide does not fully close if you dry fire it, hold it up, pull the slide back and let it slow close. Now my problem. Sunday, I shot 50 rounds and on of the fifty has a very bad case with a bulge and heavy imprint like an eyebrow where the chamber is relieved. When I slide this case back into the barrel, it will only go as far as the buldge and eyebrow on the case, which still leaves the case sticking about .020" above the rear of the barrel. It looks as though the case must have fired and was not all the way in the barrel when it did. Like maybe the slide was not fully closed and locked when it was shot. If so, it's looking like I may have to go to a slightly heavier load so I can go to a heavier recoil spring, or is their another spring that can be changed so it will fully close with the lighter recoil spring.