You could also use an Aimpoint PRO in a very low mount, but it's a bigger scope, and having used a same-size ML3 on an 870, it's not optimal, the Micro was designed specifically for this sort of thing.
I use a Micro on a rifle and they're worth every penny.
If you're stand hunting, ghost rings are just as accurate for both your purposes, i've shot standard slugs (accurately) from the prone and routinely headshotted silhouettes at 100m with a known range and no time crunch. Out to 75 yards or so under hunting conditions would be doable if you can see the target.
A red dot for a slug shotgun will improve acquisition speed on moving targets or in the dark, but it won't make you a better shot. A spare barrel with cantilever and a 2X illuminated scope would probably do better there.
Unless you train with one for a while a red dot may not be the best option for under-20 yard use indoors. At typical shotgun distances loaded with buck indoors, an XS Big Dot tritium bead is nearly as quick as a red dot (having used the Big Dot on a Glock for over a decade), where the dot wins is further out and in varied conditions, or to augment rifle sights. I've used Aimpoints on duty for years in all weather conditions and the dark and love them, but in this particular instance, if your budget does not allow it, the XS sights run about $100-150.
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