I've used an 870SM with a 28" barrel for years.
I've put several in the freezer with it, using the Remington Nitro Turkey 3½" shells and what looks like an older Primos TiteWad choke (the printing is worn off of it, I can't remember what the heck it is). I don't make shots on them over 30 yards (if I can't call these Rio's in closer than that, I need to quit). Where I hunt, the cover is extremely heavy, and often the shots are 10 yards or less.
As soon as turkey season is over, the 18.5" barrel goes on the shotgun, and it is stuffed full of #00 buckshot as a backup pig gun, and spends most of its time riding on the quad at the ranch.
*disclaimer*
I'm entirely too lazy to fret over how many pellets are in a certain area, so long as my bird falls over dead. So I have no real input on that.