Slugs work fine out of an 870.
Cheap slugs will shoot under 6" at 100 yards. RWS slugs shoot about 4" at 100 yards.
Add a rifled slug barrel and the groups shrink to 3" or less.
Brownell's sells a picatinny rail for the top of the receiver.
Add a scope on a Hastings rifled barrel with a 3-9 scope and it will shoot around 2 inches.
Slugs drop like a bowling ball beyond 100..but
We just tested slugs at 200 yards :
Federal Slugs dialed in to a 4" group, RWS shot a 3" group
and penetrated 1' of hard wood and right straight thru the door of a 2012 Toyota @ 200 yards...plenty of energy left ! 18X scope used at 200 yards.