Cleaning my gun after being at the range, I noticed that the front sight was loose. I straightened it carefully, tightened the screw, and didn't think anything of it. Then it decided to come loose mid-mag on my most recent range trip, so I tightened it again. Then it kept getting looser and looser as I shot. I think it's something to do with the barrel expanding from the heat, but I have no idea what caused this to start happening all of a sudden. The gun has several thousand rounds through it, and it's only done this the past two times I've shot it. Has anyone else had this happen to them? Any idea how to solve it?