Paul,
I have seen 870 Expresses that would throw slugs very well with the vent rib rem choke barrel. One with a bead site that would hit an empty slug boxes at 50 yards every shot. Some with clamp on rifle sights on the vent rib and one guy who uses a scope mount on the reciever with the 28" barrel. So if you don't find a slug barrel, try your long barrel.