With the recent active shooter situations that have occurred a raging debate has reignited between my neighbor and me. I carry an H&K USP45C daily and he carries a government issued Sig in 9mm (he works for one of those 3-letter federal agencies). We have debated several times the pros and cons of more bullets (9mm) versus (alleged) greater one-shot-stop percentages (.45). Without getting into religious wars about caliber, terminal ballistics, temporary and permanent wound cavity characteristics, etc., Im curious what the forum members think about how much gun and how many bullets they would want/need in an active shooter situation if they were carrying concealed. Its not easy to conceal an AR or shotgun so lets assume long guns are not an option. The old, dated FBI stats on average gunfights (take place at less than 20, 8 or less rounds expended, less than 10 seconds in total duration, etc.) dont apply in an active shooter situation so this becomes somewhat new ground for discussion on what would be the most effective in this specific situation. I carry a .45 because I know what I can hit under duress and when I do hit it, it will probably fall down with very few rounds (or the blunt force trauma when wearing a vest will disorient the active shooter long enough for me to permanently end the situation). That's my logic at least. Im interested to hear any other opinions and reasoning you may consider specifically in the active shooter scenario. And, yes, I know this is a purely academic exercise since all situations are different. Im just interested in the various approaches that people may use to come to their conclusion on what makes them comfortable.