Congressional Democrats appear as unlikely as ever to pass new gun control legislation after the deadliest mass shooting in modern U.S. history, but gun store owners say there’s one effective and easy-to-hone tool that could stop future attacks: their gut.

Gun dealers tell U.S. News they can and do refuse to sell to people they have a bad feeling about and say that fellow gun store owners could help prevent lunatics and religious fanatics from acquiring the tools needed for mass murder.

Florida Gun Supply owner Andy Hallinan has been blunt about his claimed discretion. He has a sign on the door of his central Florida shop that says “Muslim Free Zone” and last year defeated a lawsuit that claimed illegal discrimination, with a federal judge finding opponents lacked standing.

Hallinan says he has not knowingly refused a sale to a Muslim – his aim is to discourage all Muslim shoppers, he says, so he won’t have to decide if they are jihadis – but he says that he has refused sales to white men who passed background checks.

He says two men with sagging pants wanted to buy inexpensive weapons at a gun show, but he overheard them talking about “all sorts of drama going down at their houses” and told them he would not sell to them.

“I said, ‘I’m sorry guys, I will not sell a firearm to you [because] I don’t believe a firearm should be permitted in your lifestyle,'” he says. “They got all bent out of shape and swore they would file lawsuits, but at the end of the day the ATF gives me the discretion to deny guns to whomever I please depending on whatever I feel is necessary.”

Hallinan says he’s willing to refuse sales to anyone who appears nervous or gives a poor indication of why they want the gun.

Ginger Colbrun, public affairs chief for the federal Bureau of Alcohol, Tobacco, Firearms and Explosives, says licensed gun dealers have broad discretion to deny sales, such as in instances where a buyer appears to exhibit erratic behavior.

“As private business owners, [Federal Firearms License holders] can and do use discretion in determining to whom they will or will not provide service,” Colbrun says.

Suspected straw purchases – in which one person illegally buys a gun for another person – are the most common reason a gun store owner would refuse a sale and alert authorities. But the reason can also be used as an excuse.

“I can refuse a gun sale for any reason whatsoever, I don’t even have to like the color shoes you have on and I can refuse a gun sale at that point, and I don’t have to disclose the reason,” says Tim Wolverton, owner of Downrange Sporting Goods in Jackson, Mississippi.

If a gun store owner wanted to discriminate against a customer named Muhammad, Wolverton says, they could simply say, “at that point I believe it’s a ‘straw purchase,’” though even that explanation is not required.

Sometimes, gun store owners save lives by acting on a hunch.

John Downs, the owner of Downs Bait & Guns in Logan, Ohio, was celebrated in March when he refused to sell a gun to a man who passed a required background check, citing a bad feeling. He called 911 when the man returned, and police said Downs likely prevented a mass shooting.

Downs says store owners can learn about the intentions of customers by talking with them, and he advises fellow arms dealers to be vigilant.

“Just by talking to the person you get a feel of what they’re going to do with a firearm – to a point,” he says.

Downs says the government should more closely vet people who have mental disorders or potential jihadi leanings, but that “common sense” can be a substitute in the meantime.

“I hate to say I’m racist, but when you have somebody of that descent and being a Muslim and stuff, what do they want that gun for? What are they doing?” he says.

In Florida, Lotus Gunworks co-owner Robert Abell said this week his store refused to sell body armor and ammunition to gunman Omar Mateen, who murdered 49 people Sunday at a nightclub, after his questions and demeanor raised red flags.

Abell said the store informed the FBI, but did not take down Mateen’s name.

Wolverton, who once jestfully banned sales to Democrats, says that’s one thing store owners can do better.

“I did let a few get out of here when I was new at it without taking their information,” Wolverton says about past suspicious customers. “At this point, I let them fill out the whole sheet and get their driver’s license and that’s the point at which I don’t make the sale.”

At some stores, however, there are few denials of people, but for the odd straw purchase suspect.

Christopher Cole, manager of Lou’s Firearms in New Jersey, says in six years he's only had to turn away a couple people suspected of buying guns for others. He says a large law enforcement clientele may serve as a deterrent.

Of course, no amount of gun control – legislative or discretionary – can prevent all gun homicides.

Hallinan says if gun stores somehow were able to block all sales to would-be killers, weapons still can be purchased illegally, though most street items are cheap handguns rather than more expensive semiautomatic weapons like the Sig Sauer MCX used by Mateen.

Mateen bought the guns he used in the massacre at the St. Lucie Shooting Center after passing a background check. At a press conference, store owner Ed Henson said, "If he hadn't purchased them from us, I'm sure he would have gotten them from another local gun store in the area.”

Hallinan strongly disagrees, saying that at his store he recently stalled a purchase of two AR-15 parts to a “little old lady.” She was buying them for her son, which would be legal as a gift but not if he gave her money to buy them.

He says the household was cleared by authorities before he made the sale and that he’s considering setting up an association to train gun sellers to spot potential troublemakers, who despite concern about radical Muslims are often mentally ill people.

Downs, the Ohio dealer, and other store owners say sale of an AR-15-type weapon would not earn much profit and that it’s not worth taking a risk and selling one to someone who may misuse it.