No, it's not extremely rare relative to the shootings. It's extremely rare in the same way shootings are extremely rare. In a month we have had two cases brought up of civilians stopping a mass shooting.
And it is a risk, but I would say far less of one than letting the bad guys have free reign while the cops have a come to Jesus moment for 30 minutes outside. Anyone who conceal carries knows the risk. That's why, despite your belief of it being a big risk, you dont have examples of the good guys hitting a civilian. And it's why I believe it should be a crime if/when it happens, with great power comes responsibility, unless you are the government and there is no responsibility.
Remember the cops have no obligation to protect us. None. Zero. Zilch. If you are waiting on them, you are putting yourself in more danger than taking action. And that action can be running away or hiding, and probably should be. Even if armed.
Armed civilians arent sitting around waiting for the Wild West situation to start shooting up a place willy nilly. There is no evidence of it happening. You guys would parade that case around more than Trump's pee tapes. But you have nothing.