By Ali Elkin

In the wake of Thursday's mass shooting in Oregon, a familiar political pattern has emerged, with Democrats advocating for tougher gun laws and Republicans arguing that such legislation won't make a difference in curbing firearm deaths.

To make the latter point, two of the current GOP presidential candidates, New Jersey Gov. Chris Christie and Donald Trump, have invoked Chicago in their arguments, pointing to that city's tough gun laws and its high rates of gun violence as proof that the problem cannot be legislated away.

"You look at Chicago," Trump said on ABC's "This Week" on Sunday. "It's got the toughest gun laws in the United States. You look at other places where they have gun laws that are very tough, they do, generally speaking, worse than anybody else."

On the same program, Christie offered a similar argument.

"In many of the places around this country where they have the toughest gun laws, they have the highest violent crime rates. And we focus on a tragedy like this. It's an awful tragedy. It's terrible. But it is the exception to violence in America. Violence in America that's happened on our streets in our cities, like Chicago, up 19 percent, the murder rate," Christie said. "And you have some of the most aggressive gun laws in cities like that."

Chicago's high rates of gun violence have been well-documented. In 2014, there were 2,587 shooting victims in in the city, according to the Chicago Tribune. The New York City Police Department recorded 1,381 victims in the same time period, and New York has around three times as many people as Chicago.

But advocates for tougher restrictions say Trump's and Christie's arguments do not take into account two key features of the Chicago's gun landscape. The first is that, though it's hard to get a gun in Chicago, it's much easier to get one in the city's immediate vicinity. The second feature is the city's high level of gang activity, and that gangs are both adept at procuring guns illegally and prone to involvement in shooting incidents.

"I think that it's more likely that if Chicago did not have tough gun laws they would have higher rates of gun violence than they do have," said Philip Cook, a Duke public policy professor and economist who works with the University of Chicago Crime Lab and leads its multi-city underground gun market study.

Cook recently studied the origins of guns recovered in Chicago between 2009 and 2013 using data from the Bureau of Alcohol, Tobacco, Firearms and Explosives. Of the more than 7,000 guns he studied, "the great majority came from the people who were members of gangs," Cook said, adding that "the gang conflict in Chicago has been particularly lethal over the decades, and part of the reason is those organizations are skilled at accessing guns."

A lot of that access comes from outside Illinois. Cook said he found that 60 percent of guns recovered in connection with an arrest were from out of state. Twenty-four percent of the total pool of guns came from Indiana, which is "not regulated at all," he said. Chicago gangs often have connections to gangs in Gary, Ind., and the two cities almost butt up against each other.

The study also found that 22 percent of the recovered guns came from parts of Cook County outside the city, where gun dealers and gun shows are legal.

(c)2015 Bloomberg News