Hey Jake. I have the 870 fully rifled barrel that is 20" long. They also make one called a Super Slug Barrel (I believe) that is 24" long and has a matt finish. They both have scope mounts (these are Remington Barrels). I prefer the shorter barrel when sitting in treestands.
Mine will shoot three shots touching at 50 yards. I have never shot mine at 100 yards since the woods I hunt is so thick and hilly a 50 yard shot is long for me. However, based on what some friends have told me I would expect it to shoot good at 100 yards.
I like you love my 870. I left the Contender at home this year and hunted with the slug gun.
I have only had this barrel for a couple of years. Before that I had a regular IC slug barrel. The rifled barrel is much more accurate and consistent. The big disadvantage is to get the accuracy you must shoot Saboted slugs and they cost $8-10 depending on the brand compared to $2 for the old standard slugs.
Hope this helps, good luck.