Just installed a red dot sight (cheap-0 BSA) on my Marlin 39A.
I'm shocked at how accurate it is. I was lead to beleive red dots were for quick target capture, with only moderate accuracy. But dang, I can get 1" groups at 50 yards all day long. About as good as with a .22 3x scope.
A friend has one on an AR that is dead nuts on at close range like that. The problem with them is that once you get out past 100 yards, you're covering a lot of target. At 200 yds, a 4 MOA dot is covering 8 inches. At 400 yards it is completely unusable.
They're awesome for close up work where quick acquisition is paramount, but they fail really hard at greater distances.