If the gun isn't new then it might be plastic fouling in the bore. If you've regularly shot 150 grain charges with plastic sabots (or the "belt" with powerbelts) then the velocity if up in the range where fouling can occur. I had a problem with my Knight where is grouped fine then just stopped shooting decent groups at all no matter what I shot in it.

I finally get some JB Bore Paste and scrubbed out and polished the bore with it, and the accuracy returned. I just don't think that the routine hot water scrubbing does a good enough job on plastic fouling.

Mike