Just last week, I just experimented with my Mossberg 500, 18 in cyl bore barrel with bead front sight. My main use is for a bear defense gun at camp but I had never practiced with the gun.
So to burn up some old ammo and learn a bit I set up a target at about 30 feet and one at 50 yards. At the 30 ft I could keep all slugs, Remington and Federal (foster type), one Win Sabot, one Win 400gr Platinum tip, and 2 Rem Buckhammers into about a 3 inch group.
I only shot a couple slugs at 50 yds just to see how I would do. I used a 25 yd slow fire pistol targed and managed to hit the top of the black with a 6 o'clock hold with the bead. I was surprised I did so well with such crude sights and no previous practice.
I'm hoarding my last 3 buckhammers as I think they could be the best yet slug for grumpy bears. I like the 1.25 oz flat point at 1550 fps idea. They do hammer the shooter a bit too. I'm hoping a friend can find me some more in Anchorage so I can test a little more.
I'd say follow Savage T's advise about getting a gunsmith to put on some sights for you. Probably a lot more durable and affordable than a good scope and mount. I'm looking at a tritium bead since I'm thinking defense only. Or shop hard for a second barrel with rifle sights - one thing, there are a lot of accessories and options with an 870 since there's millions out there.