Went out to the desert today to relax and enjoy some of my firearms. I took 2 M1 carbines with me. One has a type 1 peep sight and the other has an adjustable sight. I was shooting the one with the adjustable sight and noticed my shots suddenly and progressively starting hitting farther and farther right. When I finally looked at the rear sight I noticed the entire sight was sliding off the rifle to the right side. It slid about half way off. I tried gently but firmly pushing it back but it didn't budge. I then pushed it harder but it still didn't budge. I didn't want to break anything so I stopped. Does anyone know why it started sliding off to the right and how to put it back into the proper position? Thanks.