From Complete Colorado:

The more people who own firearms, the harder it is to pass anti-gun laws.  Reducing the number of people who possess firearms helps with step one (reduce production), and is necessary in the long run for steps two and three.
New Jersey, Massachusetts, New York City, the United Kingdom and Australia provide the model: First, enact a licensing system. Then, as anti-gun officials make applications arduous, people are discouraged from acquiring their first firearm. Over a generation, the system greatly reduces firearms ownership.