Since the United States started the TSA and warrantless searches and wiretapping, there hasn't been a major terrorist attack on US soil.
Look, the reality is that mass murders are a different breed of people. Gun control isn't going to stop them. Nothing is going to stop them. It's like serial killers; they're determined sociopaths who will spend the time to prepare and carry out their deviant wishes.
The only thing we can hope to do with increased gun control in the US is decrease petty gun crimes: domestic violence-related murders, armed robberies, armed assaults. Virtually all of these crimes are committed by people who own handguns, and more often than not they own them illegally.
How often do you see on the news, "so and so has been arrested and charged with murder, illegal possession of a firearm"? I see them all the time. These kind of people are already possessing them illegally. Believe it or not, there are laws to prevent criminals from purchasing firearms. But there are tons of guns for sale on the black market that have been stolen from owners or shops.
Perhaps the best way to stem gun violence in America is to enact strict laws on how you store your guns--prevent guns from being stolen in the first place, and have a large gun buy-back program to get them off the streets.