Mass shootings vs people carrying guns

Has there ever been an instance of a mass shooting about to happen, but someone in the area that was carrying a weapon actually prevent it happening? I see this argument a lot in response to mass shootings and gun control laws. For example "I'd like to see them try and do that here, everyone carries a gun and they wouldn't get far" or "if only those people didn't have strict gun laws and were able to carry, then this wouldn't have happened". I've tried and failed to find an instance if this happening, anywhere.