You can shoot slugs through it. It just depends on what barrel you have on it and what slugs you are shooting.
Correct, the "rifled" (grooved) slugs are designed to be shot thru smooth bores up to and including "full" choke safely... performance will vary with choke constriction and slug chosen...
You can't shoot sabots through a smooth bore barrel.
Actually you can shoot sabot slugs in a smooth barrel and rifled slugs in a rifled barrel...
Sabot slugs are costly compared to cheap rifled slugs and you will not see the benefits of their potential in a smooth barrel but it is safe to fire them.
Rifled slugs will load the rifling with lead in a rifled barrel real quick and is going to be a chore to clean. but it is also safe to fire them in a rifled bore.
If you are planning to shoot slugs out of in go ahead and invest in a rifled slug barrel for it and shoot sabots. Then you don't have to worry about chokes
But the cost of the rifled barrel and sabot slugs may out weigh the lower cost of the "Express"... If you intend to use slugs beyond 80-100 yards, you will want to look into rifled barrel and $abot $slugs... These easily go $3+ per round... not far from the cost of .50BMG rounds... If shooting under 80 yards, buy several makes and models of "rifled" slugs and see what you and your gun do well with...
The Express is just the base model 870. They have an 870 Express Magnum and an 870 Express Super Magnum also
The express is a rougher gun planned to sell to folks shopping on price only... The Mossberg and Maverick both come in with lower price and still retain semi-nice finish...
Brent