I long ago came to the conclusion that in the end nobody knows what they will actually sacrifice their life for until they are there (as unlikely as the occurance of ever finding something like this is).
The modern ideal of heroism is big on self sacrifice, and dying for something like love, or to save the world is a noble way to end a story without getting to sappy. A lot of people see the ideal and think they would do the same thing if there, but in the end it's like any other bit of armchair heroism.
Speaking for myself I've put some thought into these kinds of things, like say if some deranged casino patron who lost everything decided he wanted to open fire into the casino video arcade and kill a kid or two at random before taking his own life to make some bizzare point in his own mind.... hey, let's just say security is boring where nothing happens, you have nothing better to do than think about bizzare thoughts.
The end result is that for all my flaws I have a family that loves me and would miss me if I died, just like the kid if I took a bullet for them. Plus despite all of the idealized potential in youth, for all I know the kid is going to grow up to be a crack whore. In the end I am not likely to sacrifice myself to save *A* kid. However if you increase the number of lives saved by such an action, I think the odds of me being willing to do it would likewise increase. "Morality by the numbers" yet again. Of course unless something like that was to happen in front of me (bloody unlikely, especially now) we'll never know, for all I know at the time I'll sacrifice myself for one kid in the moment, or lose my nerve and say "okay, machine gun that entire daycare center, I'll be heading for the back door now... kk".
As many have said, you'll never know the true measure of a man until they are tested.
Also I'm broken enough where chances are I'd probably fail any serious test of heroic fiber of any sort, and I'm honest enough to admit it.