I bought a Stag arms lower, DPMS lower kit and an A2 solid stock several years ago. finally got around to assembling the parts. Everything went well until I tried to tighten the Buttstock screw. It was the wrong thread pattern. I was told that I needed an A4 buttstock screw. the threads matched, but when the screw is tightened all the way down there is play between the buttstock and lower receiver. Stock slides forward and back about 1/16 to 1/32". What am I missing? How do I correct this? Any help would be appreciated.