from my area is old, cheap and sometimes broken guns that have probably been lying around the home for years. One thing it may help prevent is accidents from kids finding them in the house.
I don't think it has much effect on crime except for maybe a few Saturday night specials being turned in. You might get a few family members turning in guns that were or could be used by other members of the family for crime. For instance the article below mentions girlfriends of guys in prison turning in their guns.
From Boston Globe:
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The effectiveness of gun buyback programs, which became popular across the country during the 1990s, has been questioned by criminologists who have concluded that few guns used in crime are turned in.
Richard Rosenfeld, a criminologist at the University of Missouri at St. Louis, found that a buyback program there had little impact on violent crime. In addition, many people used the cash rewards for new guns and others turned in guns they no longer used while holding onto other, more favored firearms, he said.
''Gun buybacks don't have much of an impact on crime, because they tend not to attract guns from the segment of the population most likely to use them in crime," Rosenfeld said. ''City officials know they are popular, they attract attention, and they can attract attention to the overall crime problem."
John Rosenthal, a close ally of the Boston police on efforts to fight violence and cofounder of the nonprofit Stop Handgun Violence, said he does not support buybacks because they don't work.
''I applaud the mayor's office and City Hall for trying to do anything and everything, but the sad reality is Boston Police, among the best law enforcement agencies in the country . . . are never going to stop the flow of crime guns into Boston or any other city across the country until there are uniform federal laws that restrict gun access to criminals," Rosenthal said.
http://www.boston.com/news/local/massachusetts/articles/2006/04/15/city_plans_a_retooled_buyback_of_guns/?page=2