The law bans the sale of assault-style rifles, including the AR-15 used in the Newtown killing of six educators and 20 first- and second-graders. The law limits gun ownership for people with mental illness, outlaws the sale of high-capacity magazines and establishes the nation's first new handgun licensing plan in two decades.
Maryland will join five other states in requiring such licenses, a move O'Malley said "will substantially lower gun deaths."
The signing is expected to set off both a legal challenge from the National Rifle Association and a public relations campaign from gun control supporters. A petition drive is underway to stop the law from going into effect on Oct. 1, though it is unclear whether opponents can gather enough signatures in time to trigger an automatic hold on the bill. If they are, the law would go before voters in 2014.
O'Malley, a two-term Democrat, called the gun bill his top priority in what lawmakers described as the most productive legislative season in recent memory.
The gun bill is among about 250 other pieces of legislation signed by the governor Thursday, the final bill signing ceremony in the wake of the 2013 General Assembly session that adjourned April 8.
The governor also signed a landmark bill that would funnel $1 billion to repair and rebuild dozens of Baltimore schools and legislation to raise the tax on gasoline for the first time since 1992.
Other laws that received his signature extended a DNA collection database that O'Malley said led to 545 arrests since 2007, a bill that would create stricter penalties for human trafficking crimes and a capital budget that included $336 million to build schools in Maryland.
State Sen. President Thomas V. Mike Miller called the gun bill "common sense," and he offered the NRA's unwillingness to back the petition drive as evidence of the law's broad public support.
"They know if this bill went to the people, it would overwhelmingly pass," Miller said.
House Speaker Michael E. Busch said the law could be a model for the rest of the country.
Law enforcement and public officials from across the state, including Baltimore Mayor Stephanie Rawlings-Blake, crowded into the governor's conference room to celebrate the signing of the gun law. It passed with support primarily from delegates and senators in Maryland's more urban areas and drew opposition from Western Maryland and the Eastern Shore.
Baltimore State's Attorney Gregg Bernstein said the law's new fingerprinting and licensing system will help his office better track the flow of guns in Baltimore, but that the legislation did not go far enough.
"No single piece of legislation is going to be a panacea for gun violence, particularly in Baltimore city," Bernstein said, later adding, "there is much more that we can do."
Bernstein suggested another law to make it easier to prosecute straw purchasers who buy guns on behalf of someone banned from buying them, as well as a law that would create mandatory prison terms for people caught with illegal guns.
"If you've got an illegal gun, you're up to no good," Bernstein said.
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