RE: Remy 870
I'm going to assume that since you mentioned no scope that you are going to be shooting open sights. Then I am going to assume that your 26 in barrel has a double bead for sighting. Your slug barrel should have rifled sights. My suggestion would then be to go with the foster type slugs in the smooth bore slug barrel. Try a few and see which one groups best and fine tune the sights on the slug barrel from there. The rifled choke tube with sabots would be a viable option with a scope or rifled sights, but just no way to fine tune it if your impact was off your aimpoint with the bead sights.