* Fighting new laws, gun advocates cite drop in prosecutions
* 'Paperwork' gun crimes not high on Obama's priorities
* 'Using resources wisely'
A dispute erupted at a U.S. congressional hearing on Wednesday over which existing gun-control laws were worth enforcing, even as lawmakers debated whether to pass new ones.
The flashes of anger underscored the deep divisions in America's gun culture.
"You do not support background checks for all buyers of firearms?" Democratic Senator Patrick Leahy of Vermont asked the head of the National Rifle Association, the largest U.S. gun rights lobby.
NRA Chief Executive Wayne LaPierre said he did not support checking all buyers when President Barack Obama's administration was failing to enforce existing gun laws that, for example, prohibit lying on a background-check form.
"This administration is not prosecuting the people that they catch," LaPierre fired back during a hearing of the Senate Judiciary Committee, where Leahy, the chairman, began what promises to be a long congressional fight over gun control.
Obama proposed tougher guidelines after 20-year-old gunman Adam Lanza killed his mother and then six adults and 20 students at a Newtown, Connecticut, elementary school last month before turning a gun on himself.
Obama's Justice Department has shown little appetite to prosecute what it considers low-level firearms crimes at the expense of time spent on sweeping investigations, officials with the department said.
Investigators are also working under the shadow of a botched gun probe known as "Operation Fast and Furious," an investigation into gun trafficking along the U.S.-Mexico border that developed into a political scandal in Obama's first term.
Thousands of potential federal gun crimes go unprosecuted each year, the result of efforts by the Justice Department and the U.S. Bureau of Alcohol, Tobacco, Firearms and Explosives, or ATF, to determine which ones are most important, according to studies funded by the department.
The most common lead that agents pass up is a "paperwork" crime.
In those instances, someone with a criminal conviction or other disqualifying factor tries to buy a gun but does not disclose his past, either because he lied or forgot to do so. A background check finds him ineligible and bars him from getting a firearm, but he can be prosecuted for trying.
Some of those attempted buyers are otherwise law-abiding citizens who should not be a priority, said Michael Bouchard, a former ATF assistant director for field operations.
"It's simply a matter of using your resources wisely to effectively combat violent crime," he said in an interview. "To combat violent crime, they're going to go after criminals who are already in possession of guns or are using guns, rather than people who didn't get a gun."
The FBI said it conducted 16.5 million background checks for gun purchases in 2011. Of those, 78,211 ended in denials of eligibility because of a past conviction, a warrant for an arrest, drug abuse or other reason. Forty-four attempted buyers faced prosecution in 2010.
Federal prosecutors said they brought gun charges against 11,811 people in 2011, down 10 percent from their peak in 2005 when violent crime rates were higher and President George W. Bush's administration put more of an emphasis on certain gun crimes.
The National Rifle Association has repeatedly cited the drop in prosecutions as it pushes back against proposed new laws. At Wednesday's hearing, Republicans picked up on the theme.
"I hope we'll have a hearing where we'll ask administration witnesses to come before the panel and to testify why the Department of Justice and other law enforcement agencies of the federal government are not enforcing the laws that Congress has already passed," said Republican Senator John Cornyn of Texas.
Obama administration officials argue that not all gun offenses are worthy of the same attention.
Government investigators have "focused greater efforts on complex firearms investigations over the past several years in an effort to have the greatest impact on violent gun crime," said a Justice Department official, who spoke on condition of anonymity.
"While this may have resulted in fewer multiple-defendant cases, it has been beneficial for our overall public safety efforts," the official said.
Obama's own set of proposals on Jan. 16 underscored the sensitivity surrounding what gun crimes get prosecuted. Rather than ordering the 94 U.S. Attorney's Offices to devote more resources to the subject, he simply asked those offices "to consider whether supplemental efforts would be appropriate in their districts."
Obama's proposals to curb gun violence include reinstating the U.S. ban on some semi-automatic rifles, limiting the capacity of ammunition magazines, and more extensive background checks of prospective gun buyers, largely to verify whether they have a history of crime or mental illness.