Yup it's a sabot slug meant for rifled barrels--I'm no expert but have used and immensely enjoyed my 870 for several years now--the folks at Remington told me the occasional rifled slug probably wouldn't hurt the smooth bore--just don't make a habit of it. I may be wrong--but the "use rifled barrel only" has more to do with accuracy/stability of the slug in flight than a potential catastrophic failure of the barrel itself. Interestingly, from what I've read it, sounds like it's actually worse to send conventional shot through a twist barrel than it is a slug through smooth bore. interesting reading
http://theboxotruth.com/docs/bot46.htm
PS I put the ATI scorpion stock/buttpad on my 870--and I found it's great for reducing the shock loads--something to think about when firing loads with the force of an elephant gun or more.