I could not get the ARMS 18 to work with my rifle, even after I cut the center section of rail out. I still would get brass trapped between the op rod and the forward ring mount. I sold it and bought a Bassett. No problems with it.
The ARMS 18 is a great mount IF your rifle will run reliably with it, mine wouldn't, even after a lot of tinkering, changing springs, ejector, extractor, loads, etc.