870 12 gauge with a factory rifled barrel and cantilevered scope mount will shoot 2 MOA at 100yds for me. I like the smooth feel and the thinner grip 'waist' of the Remingtoncompared to the Mossy, but I own both makes.
I like to have a follow up shot in the gun, butcouldn't justify the expense of a semiauto,so I went pump action.

No need to bring your scope up above about 4x since you'll have to limit your shot distance, but get one with as much eye relief as you can find (think 4-6") and mount it up front in the rings. If you decide on a red dot with no magnification, get one with variable dot size. Up close, you'll want to have a larger dot for fast shooting, but you'll want a tiny dot (like 1-3 MOA) when sighting on a deer over 100 yards.