Have you tried the Mako slugs? Some have good success with them, but they need to be hand loaded to ensure they enter the barrel straight. Many lightweight slugs suffer from the same problem in that they have a very short length in contact with the barrel and can be slightly crooked when pushed into the barrel, especially from a magazine.
Slug size relative to your barrel diameter is also critical, you may need to slug your barrel to find out its exact diameter. The slug diameter should ideally be around .001-.002 larger than the diameter between the lands according to US forums.