This is a new one on me. I went out shooting today and had this problem: After the second shot was fired the slide would lock open. I looked closely at it and there was no bullet jam. It was as if I locked the slide back with the slide lock. At first I thought I had such a high grip that my thumb was hitting it and inadvertantly pushing it up. I repositioned my grip and it would still happen about every 5th magazine. It was not the magazine though, it did it on at least two differant magazines that I know of. It had nothing to do with rapid fire either. I fired some one at a time. Always after the second shot fired from a full magazine. The only modification I've done is replace the slide lock with the factory Glock extended lock. Could I have put it in wrong? Do I need to do some spring modifications? I haven't even field stripped it yet, I thought I'd post here first.