You, and pretty much all the other gun nuts, have this fantasy that you'll be facing the intruders or attackers at the ready, hunkered down behind a bulletproof couch or car, dispensing justice with your True American gun of BadAssery.
Here's how those things actually pan out: intruders quietly get into the house, either through an unlocked door, through an open window or through a broken window. By the time you realize what's going on, they are either pointing a gun at you, took what they came for or decided it wasn't worth it to rob an occupied house. And for every anecdote about a home owner chasing off dangerous criminals with guns, I'll give you a story about a home owner gunned down by intruders while they were looking for their gun. See for example the deaths of the district attorney in Texas.
For muggings and robberies in the street, you're looking at even worse odds, because the attacker by definition pulls the gun on you before you do. Unless, of course, you walk with your gun drawn at all times, and then you're still open for someone to surprise you from behind. And finally, to be the hero in a mass shooting, you actually need to shoot the murderer. I'd like to see you identify the right guy from a crowd of 5-6 people all pointing guns in various directions.
And for a real fun fact, I'll give you a neat robbery scenario. Friends of ours woke up one morning with all the carpets gone in their apartment (persian rugs can fetch nice sums). Turns out the criminals had actually broken into the apartment and gassed it with sleeping gas.
In short, guns are an illusion of safety. Someone who wants to get you will get you, because they always have the benefit of surprise.
And this entire business of stopping a dedicated army with rifles and guns is an even bigger illusion. Syria is nicely illustrating why.