If you're sure your sights are solid (i.e. good scope mount) the first thing I would do is switch to loose powder. I could never get good accuracy with pellets (some people do, some don't). This was a typical group with T7 pellets:
Actually, that was a GOOD group with pellets. After switching to loose powder and playing around with charges, I'm able to shoot these kind of groups with the same bullets:
Try some loose Triple Seven or Pyrodex- start at 80 grains of powder, and work your way up in 5 grain increments until you find what your rifle likes.