RE: Problem sighting in my gun.....
You have thrown in too many variables...
Yes, you should definitely shoot at one distance from a steady benchrest until you figure out what's going on...
Make sure everything is tight and nothing is moving...
Once you are sure that everything is tight and that you feel that you are shooting well ( it's easy to flinch with slugs), if the gun still does not group well, it's time to try some other brand or type of slugs..