Remember, most lone wold attackers are crazy, not marksmen. It takes lots of time on a weapon system to be proficient. Why does this matter? In most cases these sick individuals will not be as accurate and will be slow with their magazine changes, their ability to handle malfunctions, etc. For you this translates into time and better odds that as you flee you will not be fatally struck. Your odds are better of being shot if you are a sedentary target than someone egressing the scene.