I just ran 200 or so rounds though my MP .22 Compact. After firing the first magazine or so, I thought I noticed something pop off the gun. I then noticed that the there were no white dots for the rear sight. I looked around and sure enough, lying on the ground was my rear sight. I popped it back in, tightened the set screw and after 50 rounds or so, POP, it jumped back out. This happened two more times. I'll try blue loctite to keep the set screw from backing out, but before I do that, I'll take a closer look to see if there is some sort of slot the rear sight fits into and the sight isn't locking into place, but I wanted to see if I can get any feedback from the forum as to why the rear sight might be popping out besides the obvious that the set screw is backing out as a result of vibration and needs some loctite to keep it locked in place.