Many accuracy problems associated with domestically produced multi-pump pneumatic and CO2 guns can be traced to two areas. Due to the manufacturing process, particularly with guns that have the barrel soldered to the lower tubes, there are often burrs left where the transfer port enters the barrel. Another problem associated with the transfer port area is the sharp edge of the port itself. As you attempt to chamber a pellet the head tries to fall into the port. Of course it can't do that, but what can and often does happen is that the sharp edge shaves some lead off of the head as you force it past the hard spot.
Burrs have the same effect, removing lead from one side of the pellet.
Imbalanced pellets just don't shoot straight. You can determine if this is happening with your gun both by 'feel' as you chamber a pellet and by pushing a pellet either all the way through or back out the breech and examining it under a strong light with a good magnifier. The other problem area is in the crowning of the barrel. There have already been enough instructions published on the net for it to not be worthwhile to attempt to redo something that has been so well covered elsewhere. To rectify transfer port problems I prefer to use a small ballend steel burr in a Dremel tool at very low speed.