WASHINGTON — The marketplace for firearms on the Internet, where buyers are not required to undergo background checks, is so vast that advocates for stricter regulations now consider online sales a greater threat than the gun show loophole.

A new study by Third Way, a centrist think tank with close ties to the Obama administration, found that thousands of guns, including so-called assault weapons, are for sale online and that many prospective buyers were shopping online specifically to avoid background checks.

The study focused on Armslist.com – a popular classified site similar to Craigslist.org that facilitates private sales of firearms and ammunition based on location – and analyzed listings in 10 states where senators voted against a background checks compromise this spring.

At any given time, more than 15,000 guns were for sale in those states, according to the study, and more than 5,000 of them were semi-automatic weapons. Nearly 2,000 ads were from prospective buyers asking to buy specifically from private sellers, where no background checks are required.

“At this point, this is the biggest loophole in the background check system,” said Lanae Erickson Hatalsky, director of social policy and politics at Third Way.

Background checks – designed to keep guns out of the hands of convicted felons, domestic violence perpetrators or the severely mentally ill – are mandatory for gun sales at retail stores, but not at gun shows or for private sales, such as between neighbors and family members.

The National Rifle Association and other gun rights supporters have advocated against expanding the background check system because they believe doing so will not stop society’s most dangerous people from procuring weapons and eventually will lead to even stricter gun regulations, including a federal registry.

But gun-control advocates have long prioritized closing the gun show loophole, believing that is where people seeking to avoid background checks buy their firearms. Hatalsky noted that 17 states have closed the gun show loophole in their states, and that law enforcement officers have become savvy about scouring gun shows for people evading the law.

But online, she said, “nobody’s monitoring this. Nobody has any ability to stop these people who are looking for private sellers – and the only reason to do that is to evade the background check system.”

The 1993 Brady Law, which instituted the background check system, exempts all private sales, including online. Gun-control advocates say the law’s authors could not have envisioned the proliferation of online gun sales.

The compromise struck this spring between Sens. Joe Manchin, D-W.Va., and Pat Toomey, R-Pa., would have required background checks for online sales, but the legislation fell short of passage.

Third Way hopes its study will illustrate to senators who voted no the magnitude of online gun sales in their states and help convince them to change their positions if the legislation comes up for a second vote.

The study is a snapshot of sales on Armslist.org in June and July. In Georgia, for instance, there were 1,960 firearms for sale, while there were 6,192 for sale in Ohio and 124 for sale in Alaska.

In some states, many private sellers advertised numerous weapons for sale at the same time. One person in Arkansas had 22 separate guns listed for sale, while individuals in Nevada and Ohio had 21, according to the study.

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