Hundreds of high-volume private gun dealers are transferring tens of thousands of firearms every year over the Internet without conducting background checks, according to a report from the gun control advocacy group Mayors Against Illegal Guns based on an investigation conducted from August to October.

The report was released as gun control advocates try to keep pressure on a divided Congress to work on legislation increasing the checks. Nothing was accomplished this year despite heavy lobbying from the White House and other groups in the wake of the Connecticut school shootings.

The investigation looked at listings on ArmsList.com, a firearms clearinghouse akin to Craigslist, where prospective buyers can check out supplies posted by individual sellers.

The investigation found that “high-volume sellers” — those who listed five or more guns — posted 29 percent of the private seller gun ads on ArmsList.com during the period, a rate at which the sellers would transfer 243,800 guns each year.