I had a gen 4 19. The dual coil spring got locked up through the guide rod hole and nearly permanently locked the slide back. It took a lot of beating and prying to get it loose and I ended up deforming the recoil spring. After paying $40 to have it replaced, I traded it in on a different gun. Not sure if this is a common problem with the gen 4's but it happened to me. Kindof ruined my own little glock experience.