I've got a Remington 870 Express Super Magnum (3.5" chamber). Does anyone know if "they" have started making 3.5" 12 gauge slugs yet?

Lightfield makes a 3 1/2' sabot slug (1 1/16 oz. slug (465 gr.) at 1900 fps). I doubt the sabots would shoot very well with the smoothbore barrel though. Unless you want to load your own slugs I would go with Brenneke slugs.

Recoil would be heavy!