I'm not sure I agree. In France, for example, "all semi-automatic rifles with a capacity greater than 3 rounds, all handguns and all rifles chambered in 'military' calibers, including bolt action, require permits." (Wikipedia) The vast majority of citizens not only follow the law and obtain permits should they want such a firearm and France has a much lower rate of firearm injury and death than we do. 
At the same time, when terrorists wanted to get similar type weapons to kill 150 French citizens they were able to do so because it is very difficult to stop people who are determined to do harm.
We might not be able to prevent mass murder in all cases but it could be greatly reduced with reasonable firearm regulations. Clearly it's only a matter of time before the US adopts regulations that slow the flow of casual purchases of semi-automatic, high-capacity weapons. Citizens unwilling to put their names down and register with the state won't be able to purchase them, I predict.