75yds. is about tops with a smooth bore slug barrel w/sights, unless you want to consider how lucky your are. You can take it further if you top your 870 off with a good quality scope and spend time at the shooting range shooting a variety of slugs to find what works best for you.
After I bought a Hastings rifled slug barrel and scoped my 870, I was able to harvest deer past 150yds. without any diffuculty, but you need spend time at the range to find what your limits are.
I just recently purchased a Remington 23" heavy rifled slug barrel and I've tried 2 different brands of slugs for rifled slug barrels and neither of those 2 slugs worked in this barrel. In another words, no matter what you have for a barrel you need to buy different brands of slugs to find what works best. You can not go by what your buddies shoot....do your homework before you go hunting.