There are more guns in circulation than there are American citizens
But it's not what that datum suggests. Fewer and fewer people own guns. But those who do own guns, own more of them. Hence, shooters tend to be well-armed with multiple weapons and lots of ammunition.
and yet gun proponents still cling to the "good guy(s) with a gun" being present as a credible defence against a "mass shooter" armed with an AR-15!
It's not a totally crazy idea, but consider. A "good guy with a gun" is minding his own business, on a street one night, when he sees a black guy pull a gun on another person.
What should good guy do?
Oh, forgot to mention, black guy is a police detective, apprehending a dangerous criminal. Was the first decision a good one?
Suppose you're in bed one night, your spouse with you, and with a gun at your nightstand. You live in a high crime area. Suddenly some guys suddenly break into your bedroom through windows. At least one of them is armed. What do you do?
Oh, forgot to mention. The three guys were police officers who messed up on an address and were doing a "no-knock" raid on the wrong house(they had a warrant for the right one). Was the first decision a good one?
This one actually happened recently. The homeowner (who happened to be black) shot the three cops. Remarkably, the survivors didn't kill him, but arrested him for homicide.
But the "good guy with a gun" idea isn't completely crazy. Few decades ago, the National Safety Council did a study of guns and NRA stats on people who used a gun to shoot or scare off an assailant, and showed that about 20% of Americans live where it's dangerous enough that having a gun is actually safer than not having one. Given the drop in violent crime since then, the number is almost certainly smaller than it was.
So there is that.