WASHINGTON -- Wal-Mart WMT, the nation's largest seller of firearms, announced Monday it will toughen rules for gun sales, from storing video of purchases to creating an internal log of which guns they sell that are later used in crimes.

J.P. Suarez, the chief compliance officer for Wal-Mart Stores, appeared with outspoken gun control advocate Mayor Michael Bloomberg of New York to announce the changes at a gathering of Bloomberg's group Mayors Against Illegal Guns.

Changes to come at about 1,100 Wal-Mart stores selling guns include:

• Creating a record and alert system to record when a gun sold at Wal-Mart is later used in a crime. If the purchaser of that gun later tries to buy another gun at Wal-Mart, the system would alert the sales clerk of the prior buy and could refuse to make the sale.

• Retaining the recorded images of gun sales in case law enforcement wants to view them later as part of an investigation.

• Expanding background checks of employees who handle guns and expanding inventory controls.

Suarez said the tougher standards will come with some additional cost to the company.

"The costs are we think part of what it takes to be responsible. Everything is not pain free," he said, adding that small sellers can implement many of the same rules. He did not say how long it would take to implement all the changes, but noted that software must still be created for an internal log of guns later used in crimes.

Suarez said his company may receive some pressure from gun rights groups, but added, "This is not a signal that we're getting out of firearms."

Bloomberg urged other companies to join Wal-Mart in the initiative called the Responsible Firearms Retailer Partnership.

"We didn't pressure them, they're doing it because they think it's the responsible thing to do," said Bloomberg.

Wal-Mart, the world's largest retailer, had previously tried to establish a store in New York City but failed.

The mayors' gun summit also unveiled a new lobbying effort to close what they call the "gun show loophole" that allows people to purchase guns without background checks at gun shows.

Bloomberg founded the group two years ago with Boston mayor Thomas Menino to reduce the flow of guns from store displays into the hands of criminals.

The group, largely funded by Bloomberg's personal fortune, announced it was spending more than $100,000 on television ads starting Wednesday featuring all three of the current main presidential candidates voicing their opposition to the gun show loophole.

The ads will run in the home states of the three presidential candidates — John McCain of Arizona, Hillary Rodham Clinton of New York and Barack Obama of Illinois. It also will air across Pennsylvania, which holds its primary next week, as well as Florida, Maryland and Massachusetts.

The mayors group is trying to gather support in Congress to:

• End the gun show loophole.

• Require gun dealers to perform criminal background checks on all gun-handling employees.

• Close a so-called fire-sale loophole that allows gun dealers whose licenses have been revoked by the government to sell off their inventory without background checks.

• Add those placed on the terrorist no-fly list to the list of people prohibited from purchasing a firearm.