I got a GG&G receiver sling mount for my AR with a shorty A2 stock (Rock River Entry Tact stock)
It came with the mount, a screw and a ring. What is the ring for? I installed the mount and used the GG&G screw to replace the top buttstock screw, but couldn't figure out where the ring goes. Also, after installing the sling mount, the rifle will not let me lock the bolt back with the catch so I removed the sling mount now the catch holds fine. Any advice appreciated.

I received an answer. Since mine is a stubby stock the bolt wasn't traveling far enough back to engage the catch. The screw is too long and needs to be cut down when using a stubby stock. The ring is a spacer that long stocks need to use.