Alas, the answer to most of these questions is 'it depends.' And then when we get to the what-might-have-been questions, things get even more speculative.
But it still may be instructive to look at a couple of high-profile incidents:
In the Aurora, Colorado shooting, the killer was a methodical, well-prepared lunatic. That's an unusual combination, so there probably isn't much that would have stopped him. Deny him a legal gun, and he could probably find an illegal one. Deny him that, and he could build a bomb to throw into the theater. Deny him that, and he might build a truck bomb. And so on.
In the Gabrielle Giffords shooting in Arizona, the shooter did not put as much effort into preparation. Here, regulations could have changed the equation. In particular, if he hadn't had legal access to a 30-round clip, it's unlikely that he would have had one. Then, there probably would have been fewer bullets fired and fewer people shot.
That's not to say that we should ban 30-round clips - there are other incidents and a number of other trade-offs to consider. In this case, there are already so many in clips circulation that it's unlikely that a ban would be very effective.
But we definitely can talk about it.
(PS - in the middle of our discussion, this post was added. I think he just wanted to demonstrate how to make an infantile post about the issue :-).