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-check 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 purchase 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, an organization that has been active in the gun-control movement for years.

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 or between individuals online.