I recently put a scope on two 22lr rifles; a little Marlin tube fed semi-auto and a single shot Chipmunk for my grandson. The scope won't stay put on either of the two rifles. The recoil walks the scope forward on the mount until it comes off. I've been shooting for over 40 years; I know how to mount a scope, I've done this before. Has anyone else run into this problem or better yet has anyone else found a solution?