Plastic fouling from sabot?
Gun was made in Turkey, so possibly a rifling issue.
I shoot the Winchester BRI sabot slug from my Rem 870 Sureshot Turkey with cantilever rifled barrel.
I boresight at 25/50yds and hit 8" circle at 100yds then fine tune.
I've never boresighted and gone straight to 100, always shoot a couple at 25, 50 then 100.
I have Nikon Slug scope too.
And these ones I'm giving try this fall;
Slug R Us - "Hammerhead Slug"
Hope it all eventually works for you, there's nothing like sailing a slug thru a deer!!!