Karl Durkheimer is more coy than might be expected of a man with a pistol on his hip.

Durkheimer's gun shop looks to have enjoyed record sales of semi-automatics – "modern sporting rifles", as he calls them – and handguns at the weekend. But he doesn't want to talk specific numbers.

Nor is he terribly keen to speculate on the causes of the sudden demand. But he acknowledges that it probably has everything to do with a man killing two people with an assault rifle last week in a Portland shopping mall less than 10 minutes drive away, and Friday's massacre of 20 small children and seven adults on the other side of the country in Connecticut.

"Handgun sales are up substantially and modern sporting rifles are up astronomically," he said after a few days when his shop, Northwest Armory, was packed with buyers sizing up the most popular pistol in the US, the Glock, and the military-style AR-15 assault rifle, which also comes with a pink stock for women. "The people you see are twofold. There are first-time buyers who are in fear of what the future will bring. But most of what you saw is people hedging their bets that there might be a political policy put forward by the liberal side of the government."

Durkheimer means his shoppers fear that the shock of the Newtown, Connecticut, killings might cause the public and Congress to support a reinstatement of the ban on some of his most popular lines.

That's a picture replicated across the US from California to Louisiana, and even in Newtown where Robert Caselnova said his gun shop saw high demand for assault rifles in the days after the killings. The nationwide increase in sales was reflected in longer than usual delays for legally required background checks which in some cases took hours rather than minutes.

The surge in sales is not unusual. Following a mass killing at a Colorado cinema in July, applications to buy guns rose more than 40% in a week. The murder two years ago of six people during an assassination attempt against congresswoman Gabrielle Giffords, who was badly wounded, prompted a 60% increase in gun sales in a single day in Arizona.

The increase in weapons sales also comes in a year in which the FBI reported a record number of background checks for gun purchases, with nearly 17m applications.

Durkheimer has seen that reflected in his own sales which have been rising all of this year. Last month, with Barack Obama's re-election, they were more than double those of December 2011.

"No question, an election year where Democrats are winning drives a lot of business," he said.

Particularly when it's a Democrat like Obama who many conservatives regard as somehow un-American and fear he is after their guns even though his four years in power have seen only a loosening of firearms regulations.

But Durkheimer, who counts elephant hunting in Africa among his pastimes, has not seen anything like the past weekend.

"There are two kinds of Americans. People who want to take care of themselves and those who want to be taken care of. The ones who want to take care of themselves are the ones who come into my shop," he said.

The buyers regard the Newtown killings as a tragedy, but view any connection to their right to own weapons as a political ploy aimed at depriving them of their guns.

"It's terrible what happened. It's just plain evil," said Richard Merritt on the steps of the gun shop after browsing assault rifles with a thought to buying himself one for Christmas to supplement the handguns and hunting rifle he owns.

"But there's people trying to use that to say I'm responsible because I own a gun. Where's the connection? The only people making one are doing it for political ends because there's not one of these massacres would ever have been stopped by a law that takes my gun away. But now they're talking about doing that again, I think this may be the time to buy."

Karl Durkheimer has seen sales in his gun shop increase throughout 2012. Photograph: Chris McGreal

Durkheimer is sick of gun owners being painted as the problem. Like many, he feels demonised and vilified for the crimes of a few because he enjoys hunting and shooting ranges. That puts him on the defensive when groups campaigning for tightened gun regulation might be better off trying to win him over with assurances that their calls to restrict the sale of assault rifles and magazines that hold large numbers of bullets won't end with the confiscation of handguns and hunting rifles.

"It's just another thing that will drive a wedge between us. Instead of the United States being a melting pot it's more polarised," he said.

It's a common claim among gun owners that states with the greatest restrictions on weapons are facing rising crime while gun crime is falling in those with the loosest laws, such as permitting the carrying of concealed weapons. A study by the Center to Prevent Gun Violence in California found the opposite to be true: that seven of 10 states with the strongest gun laws, including Connecticut, are among those with the lowest rates of deaths from guns.

Then there's the conventional wisdom among gun owners that if only someone at any of the recent massacres had been armed, they could have put a stop to it. That was the position of Gun Owners of America, which said that if teachers at the Newtown school had been armed, the killer could have been stopped.

Its director, Larry Pratt, went so far as to say: "Gun control supporters have the blood of little children on their hands".

But just down the road from Durkheimer's shop, at least one person carrying a gun during the shooting at a shopping mall last week decided not to fire it at the man who murdered two people before shooting himself. Nick Meli told the Oregonian that he trained his weapon on the killer, Jacob Tyler Roberts, but didn't shoot because he was worried about hitting an innocent bystander.

There is one area of common ground for Durkheimer. Demands by some political leaders for greater gun control have been accompanied by calls for increased scrutiny of the all too common role mental illness plays in massacres, and what that means both for better treatment and for weapons sales. Although that's not exactly how Durkheimer sees it.

"The black and white is the guy in Connecticut's a very sick person. What kind of a person will do harm to his mother?" he said. "I believe this problem, this tragic event in Connecticut, shows we have a huge mental health and social services problem. The do-gooders in society think it's unfair to institutionalise people so we're going to make a concerted effort to integrate them into society. And then they are loose."

Does he sell guns to those who appear to be mentally unstable? "I definitely have seen people come in and say some things," he said, trailing off. "But I believe firmly it's not my place to be judge, jury, God on who has a gun."

Durkheimer is confident that calls for greater control won't go anywhere at a national level. For many gun owners, he says, it is a voting issue.

"When Bill Clinton introduced the assault weapons ban, it cost the Democrats a hundred seats in Congress. Since then, you haven't heard anything about gun control until this shooting. The Democrats have been quiet. I guarantee you, behind closed doors elected Democrats are saying: I don't want to talk about guns," he said.

"Hypothetically we could have some kind of presidential executive order. But I don't believe there'll be any kind of vote between the Senate and the House of Representatives. I think there could be some states that pass laws. Oregon could be one of them. There are a lot of displaced Californians here who have left their state going to the potheads and down the toilet, and now they want to do the same here."

But he wryly notes that the Taliban have kept the American army busy for a decade precisely because Afghanistan has a long standing culture of every man having a gun.