As J.D. Tuccille has noted here regarding Connecticut, people don't tend to want to give up weapons they already own just because it is now technically illegal for them to have them.
Maryland is contemplating testing this out when it comes to people legally barred from gun ownership because of their criminal records, as Associated Press reports via CTPost.com:
The House of Delegates is considering a bill that would link Maryland's gun registry to its criminal database.
State law requires gun owners to surrender their weapons if they're convicted of felonies or any violent crimes, but Del. Luiz Simmons, D-Montgomery, said Maryland State Police lack a systematic way of enforcing this.....
State Police estimate that if they linked the databases, they would find 10 percent of registered gun owners — about 110,000 people — would be disqualified. They estimate a rate of 1 percent each year thereafter.
Under Simmons' plan, police would run a systematic check at least twice a year.
The Department of Legislative Services believes it would cost about $300,000 to create the database. The department also anticipates a need for seven new full-time troopers to investigate findings from the new database, with a cost of more than $1 million a year for salaries and equipment...
Several independent gun advocates submitted written testimony before a Tuesday hearing. They argued against the measure, calling it unnecessary and potentially invasive.
"This smacks of a police state mentality," said a resident named Ronald Smith.