Horrible tragedies, such as the one in Colorado last week where a mass shooting happens, often have one noticeable effect. Guns sales immediately skyrocket — in this case up 43% over a week’s period in Colorado — after an event due to people who become concerned that new gun control measures may be put into place and make it difficult or illegal to buy the gun of choice they wish to have. This happened immediately after the Giffords shooting as well. Why does this happen?
People fear that the government will take away their rights to buy and/or own a gun. They fear that the government will use the mass shooting event to implement new policies that they don't support because of the emotion involved. Why go out and buy a gun though if you fear the government will make that gun illegal? You only need to look to past gun control measures to answer that question.
Past gun control measures that have passed, notably the Firearm Owners Protection Act and the Federal Assault Weapons Ban, were non-retroactive. That means the bans started the day the law went into effect and did not effect current ownership. The Firearm Owners Protection Act banned automatic weapons that we not registered prior to the bill going into effect. That meant that all machine guns made and registered prior to the bill becoming law were legal (and still are legal) to own. It only banned machine guns made after the bill became law. Similarly, the Federal Assault Weapons Ban did the same thing with so called assault weapons.
Knowing this, people naturally believe that any future gun control measure will also be non-retroactive. That explains the rush to buy guns after a mass shooting. People want to get their guns before it becomes illegal to do so should a new measure pass.
The kicker is, Congress could pass a retroactive gun control bill (in theory). Of course they would have to have some way to enforce it. Likely that would mean confiscating people's guns that have been deemed illegal. Somehow I doubt that will ever get majority support from Americans or pass constitutional muster. The fact is that America was built by gun-owning rebels who decided to overthrow a tyrannical government from abroad. Gun ownership is part of American culture. It always has been and likely always will be.