Are you sure it's the ris rail and not a wobble between upper and lower receiver? I've had similar wobble that turned out to be receiver wobble. Some ris rails also have a tightening lever thing that allows you to tighten them to the delta ring. If there's a screw on the top of the rails that can tighten this lever. (picture from an lct ris rail with this lever)
But all that aside if there's a wobble then there's space somewhere and you need to find it and remove it. Best of luck!