One, I never said to ignore wind resistance. Reread and you'll see that. Two, when speaking about mathematic prediction I assumed tumbling. No one ever specified a bullet type, so I went with what I believed to be a general answer. A hollow base bullet- like an SS109 M16 bullet (one I know REALLY well) would most certainly tumble when falling backwards, greatly reducing its rotational velocity (at least in the original vector). My point was that there are far too many variables to predict where the bullet will land and how it will be pointed when it hits. Bullet shape, crosswinds, temperature, altitude, and so on and on make an accurate prediction impossible- a three shot "group" MIGHT cover a three block radius. THAT'S why I said this isn't a calculus problem, but one for statistics. I'm sorry if I came across confusing. Believe it or not, I was trying to keep it relatively simple. As for whether it would be lethal on impact- I wouldn't want to be hit by it.
Anyway, I think we've veered off the original thread (retained lethality after going through a wall) and I'm guilty of helping push it that way. As such... I'll apologize to the OP for helping hijack his post. SORRY.