Kleanbore said:
It is just as easy to shoot up the stairs as down
Without attempting to draw any conclusions from the below, I would say that this is pretty demonstrably untrue. It's untrue for the same reason that (even though I don't agree with it as often as most people do) people constantly repeat "don't clear your house."
A dominant position is a dominant position. Stairs with a typical angle and of a typical length do allow for greater visibility with less exposure, at the top relative to the bottom. This advantage can be accentuated or conceded by the person at the top.
Even if a person entering a doorway at the bottom of the stairs is aware there is an armed person inside, there are other areas he/she must direct his/her attention besides the top of the stairs, which are not in immediate view - it's not normal to enter a room/door with one's head canted back at a 30 degree angle.
This is why 3+ "tactical football players" (SWAT team members) enter rapidly and fan out in a situation like that. The room is pre-divided geometrically - one guy's "up," another is "left," another is "hard corner left," another is "right," another is "hard corner right," another is maybe backing up the "up" guy.
That's the only way highly trained guys that train monthly or weekly can afford to assume "He is at the top of the stairs!"
Chances of a bad guy a) knowing stairs are there, b) assuming defender is there, c) being correct in any given instance (when there are usually, as I hopefully demonstrated, at least 7-8+ planes of concealment), and d) if we still assume for the sake of argument that all of the above is true,
getting off an accurate shot first or having the wherewithal to withstand incoming fire from the defender...well, I don't know chances, but I'd disagree with "just as easy."