I took Karate myself, and I could punch you and knock you out before you could get a spinning back kick off. They are nice to look at, but not quick enough in an actual fight.
One punch knockouts are extremely hard to accomplish. In fact, I have never seen one, not in all the boxing, MMA (mixed-martial art) fighting, and street fighting I have participated in or watched. I have seen very powerful fighters effect knockouts with a small number of punches...three or four in succession...guys like Tyson in his prime, Vitor Belfort against Tank Abbot.
I used the spinning back kick in the example because that's what I happened to have been throwing for warmups when the guy commented. I myself am much better with my hands than my feet, and recently am probably slightly better grappling than I am with my hands. But, that said, I have landed spinning back kicks, and even jump spinning back kicks, in very aggressive MMA sparring matches, and have seen a surprising number of spinning kicks attempted successfully in professional MMA and no-holds barred fighting. It just isn't true to say that they aren't quick enough to be used in a fight. Coming in on a straight trajectory, they are actually very quick kicks, and much harder for the opponent to see than, say, a big Van Damme-esque spinning cresent or spinning hook. They are actually the only spinning kick I bother to use.
In fact, it's also not necessarily true that you could punch me before I could get the kick off, were we in a real fight. If you were right next to me, sure. As with any kick I'd have to create separation first, and by then your punch would be thrown. But if you were 5 feet away to begin with? Maybe now, not having thrown that kick in almost two years, I might not be fast enough anymore. You might get to me first. But back when I was training regularly? Nope. I know, because we trained to kick people who were rushing in, that I could get the kick off before you could reach me with a punch. That doesn't mean I would automatically connect, but at the least it would make you drastically change your own trajectory.
Of course, it's best not to just throw one out there as a stand-alone technique. But it's best not to throw *anything* out there as a stand-alone technique, kick or punch. You need to set it up properly, just like any technique.
But that's all beside the point, as is the distance between the unarmed fighter and the guy proclaiming the superiority of guns. I have taught self-defense classes for years, and I always stress that almost nothing you have is worth being shot over, and that, no matter how fast you are, you cannot close a 5 foot gap without being shot. Do I know how to take a gun away from someone? Yes. If they are close enough, I know a number of fairly efficient ways to do it. What are the odds I would actually succeed? Who knows. Not me.
But my point in the story above was that the guy, claiming confidently that he could shoot me before I could kick him, DID NOT HAVE A GUN ON HIM.