Reality is that a single semi-automatic rifle would have been extremely effective in stopping the madman in Nice attack.
I don't know how long it took that guy to plow through the crowd but I am quite positive no one in that crowd would have had the time, situational analysis, and space to end the threat without taking out more innocent bystanders.
I'm not anti-gun, there are just very few people trained to respond with deadly force in traumatic situations. Your average citizen is not one of them. Your average citizen is going to protect their own life by getting the fuck out of the way.
"In order to make an apple pie from scratch, you must first create the universe." -- Carl Sagan, Cosmos