I've played out these scenarios in my head quite often. I like to think that situation dependant, I would have the gonads to confront the shooter with my barehands if no escape or other tools of violence were available. One thing I try to live by when dealing with threats was taught to me by some of my former NCO's. "Speed, surprise, and violence of action...' as well as situational awareness. If it's a guy around my size and is with in close proximity, I *think* a good bear hug from behind and tossing him back on his head might disrupt him enough to at least get the gun out of his hands. Pressure points and soft spots would be a quick target. *IF* if I manage to get him to the ground, a good elbow or punch to the nose might increase these odds. Things to consider however is what direction is the shooter in relation to me, what is the intent of the shooter, and what environment is this in? For me personally, a lot of it dependant on whether or not there will be collateral damage from my actions, or lack thereof. Will your actions if failed escalate his force against others? I value my life very much, but I also value others. If it's a daycare or something, you can bet your sweet cheeks I am going to try everything in my capacity to do something. If it's in a food court in a mall and I'm 4 stores down theres not much of a chance me running towards the guy or even trying to sneak up on him will work.
Another thing to note. What is he using as a weapon? Manipulating a rifle/shotgun out of his hands while keeping the gun not pointed at you would certainly be easier than a handgun.
This is ALL pure speculation on my part. There are so many ways to cut this pie, and I honestly have no idea how I would react in a given situation. At the very least however, contemplating it is a form of preparation.
"In 1968 for my senior field trip I was sent to RVN"
"It is far more important to be able to hit the target than it is to haggle over who makes a weapon or who pulls a trigger." -Dwight D. Eisenhower