WASHINGTON — A private seller from Pensacola, Fla., is offering to trade an AR-15 semi-automatic rifle for a jet ski or a riding lawn mower. Another in Huntsville, Ala., will barter ammunition and magazines for an iPad Mini.

The online postings this month on Armslist.com, one of the busiest gun-swap websites, are part of an underworld of firearms sales thriving in the United States after senators in April failed to advance a measure requiring background checks for private gun purchases.

“It’s a loophole so large you could drive a Mack truck through it,” said Rep. Jackie Speier, a California Democrat who served on a House panel on gun violence. “There’s a way to get around” the existing state and federal firearms laws, she said.

Gun-control advocates including a super-political action committee founded by former Arizona representative Gabrielle Giffords, who was shot in the head in 2011, are regrouping 15 months before the 2014 congressional elections. To muster public support for stricter gun laws, they’re running ads and visiting the states – including Arizona, New Hampshire and Arkansas – of senators they want to persuade.

The groups are tailoring their message to what they see as the most glaring legal flaws, the rise of a largely unregulated private firearms market on the Internet and at gun shows. Purchasers have found ways around background-check rules, and private sales have flourished. While sales across state lines must go through a licensed dealer, in many states trades without such checks are legal within a state’s borders.

Studies to be released this month will emphasize the increasing number of private gun deals arranged over the Internet. Third Way, a Washington research group aligned with Democrats, found more than 15,000 sales ads on Armslist.com in a recent survey.

Another report obtained by Bloomberg News, by Giffords’ Americans for Responsible Solutions, said that more than 67,000 guns were for sale from private sellers on Armslist.com., where sellers can list multiple guns in one post.

Gun-control advocates pushed for tighter laws after the Dec. 14 shooting of 20 first-graders and six adults at Sandy Hook Elementary School in Newtown, Conn. Those efforts fell apart with the April 17 Senate vote that thwarted an expansion of background checks. Advocates say they aren’t giving up, touting public support for stricter gun laws – 90 percent of respondents in some opinion surveys supported background checks.

Still, little appetite has re-emerged for gun legislation in Congress. When lawmakers return Sept. 9 from a five-week recess, they will be preoccupied by debate over funding the government and raising the nation’s debt ceiling.

Earlier this month, Senate Majority Leader Harry Reid, D-Nev., told the group Moms Demand Action for Gun Sense in America that he will schedule another vote on background checks no sooner than next year, before the midterm congressional election.

Most gun-restriction measures are opposed by Republicans, while Democratic supporters haven’t won over colleagues facing a potential backlash from the gun lobby.

“There’s not going to be any policy change until there’s a different composition in Congress,” said John Hudak, a fellow at the nonpartisan Brookings Institution in Washington.

The groups, though, are heating up their campaigns. Mayors Against Illegal Guns aired television ads July 20 marking the one-year anniversary of the shootings at a movie theater in Aurora, Colo., where 12 died and 58 were injured, featuring survivor Stephen Barton.

The ads ran in the states of senators who didn’t support the background-check bill. The group was co-founded by New York Mayor Michael Bloomberg, who is founder and majority owner of Bloomberg News parent Bloomberg LP.

The mayors group is in the midst of a bus tour to 25 states over 100 days, with gun-violence survivors and their families, law enforcement and local advocates. This week, the tour was in Ohio, which had 6,192 guns for sale online by private parties – the largest number among 10 states surveyed, according to Third Way.

Advocates are raising money for another legislative push. Mark Kelly, a former astronaut and Giffords’s husband, predicted that the seven-month-old, Washington-based Americans for Responsible Solutions, which has taken in more than $11 million, will raise more than the average $18 million to $20 million that the National Rifle Association spends per election cycle. The NRA is the nation’s biggest gun lobby.

“This is an area where a lot of people can coalesce, even if they say they’re opposed to gun-violence prevention,” said Sen. Richard Blumenthal, a Connecticut Democrat who’s joined with Newtown families in pushing legislation. “There is a core group of senators, including myself, that is determined to seek another vote.”

The 1993 Brady Handgun Violence Prevention Act sought to require background checks for most firearm purchases by mandating screenings for all commercial sales at gun stores.

The new data on gun buyers evading background checks over the Internet is fueling the advocates’ efforts.

“The center of activity for prohibited purchasers has clearly shifted to the Internet, for obvious reasons,” said Mark Glaze, director of the mayors’ group. “No background check, no questions asked, no paper trail – all in the dark.”

The Third Way study found 15,768 sale ads listed by private sellers on Armslist.com. Of these ads, 5,136 postings were for semi-automatic weapons, including assault weapons, and 1,928 of the ads were from prospective buyers looking to purchase firearms from private sellers, the report said.

“It’s really scary when you look through the online want-ads seeking private sellers,” said Lanae Erickson, a policy director at Third Way. “Why would you need to buy it from a private seller unless you are trying to get around the background check?”

The Third Way analysis covered gun sales in 10 states during June and July. These are states represented by senators who had been sought by anti-gun violence advocates as potential supporters of expanded background checks, including Republicans Rob Portman of Ohio, Kelly Ayotte of New Hampshire, Jeff Flake of Arizona, Johnny Isakson of Georgia and Democrats Mark Pryor of Arkansas and Mark Begich of Alaska.

This week’s report by Giffords’s group documents the case of a man with Asperger’s syndrome from Nevada who ended up purchasing a gun from Reno police Sergeant Laura Conklin on Armslist.com. According to the man’s mother, Jill Schaller, who reported the transfer, it “put her family in danger and could have ended her son’s life,” the report said.

A sampling of Armslist.com postings shows the ease with which weapons are obtained.

Trades occur with the click of a mouse and then a meeting. One listing for an M44 carbine bolt action rifle highlighted by Third Way advertised: “You can avoid the background check and taxes by taking this rifle off my hands for $250.”

Perry Tarrant, a police captain in Tucson, Ariz., who has worked in law enforcement for 33 years, said the Internet has compounded the gun violence in his state.

“Historically, we’ve always thought of Arizona as being the Wild West and a very strong gun culture,” he said in an interview. “The Internet is an even wilder West. Behind a computer, the origin of a seller can be anywhere around the world.”

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