308 Won’t group?

I have had this .308 for some time now. I have tried several scopes, several shooters and tried every type of hold to see if there was a way to improve it. I can get 3 shots real close at 50 yards then next one flies almost off the page. Gun is tethered to a Ninja bottle with 2650 psi output, pellet is JSB 50.1 gr at 900 fps. I tried lower and higher fps. Barrel cleaned, leaded barrel and same problem. Also sent it back to Airgun Source, all I was told was to tether it, got same results.