I don't think there's nearly as many as you think. It's a perception bias, the few that there are stick out a lot because they are hilariously stupid so you read about them a lot and it seems common.

Keep in mind that in the US, about half the households are armed. And the half that own guns own enough guns to arm the other half. There's more guns than people in this country. If they're truly was a significant overlap between very stupid people and gun ownership, the nation would be like a roadrunner cartoon with Yosemite Sam type a shootouts and people firing into the air on every street corner. That is seriously not the case.