They could be hidden in the back of a tractor trailer, stuffed inside the panels of a car door or simply tucked into a bag of luggage.
While it's difficult to track, police estimate that only about five per cent of all firearms that are smuggled across the 49th parallel are ever caught by customs officials.
One way they're fighting back is with NWEST, the National Weapons Enforcement Support Team, an action group made up of members of various police forces.
Formed in 1997, NWEST uses computer networks and the national Firearms Registry to tracks guns used in the course of criminal activity.
Officers cross-reference serial numbers from handguns and try to track the path of a gun from buyer to seller and beyond.
Geoff Francis, NWEST's deputy director, says it's often tedious work, but it's well worth it.
"If you can get somebody who supplies hundreds of guns to five different dealers and take him off the street, you've stymied a source," he explained.
Francis says that while fighting gun smuggling from behind a desk may not seem as useful as battling criminals on the street, it can often prove more productive.
Such detail work can often steer investigators directly to the gun suppliers, say some Toronto police officer.
"I think we've had an impact," said Det. Steve Horwood with Ontario's Provincial Weapons Enforcement Unit. "We identify the traffickers and I think we've taken a lot of action which has closed down the sources."
"The one thing that's not measurable in this business is how many lives you save by taking out that trafficker in the U.S."
In addition to this, police in Toronto will soon get a big U.S. boost in their efforts to curb smuggling.
The Department of Alcohol, Tobacco and Firearms, the American government agency that deals with firearms, has assigned an agent to Toronto.
The ATF already has two liaisons officers in Canada - one working out of the American embassy in Ottawa, the other in Vancouver. It's hoped the additional resources will help authorities in the Toronto area deal with smuggling.
"We believe in a city the size of Toronto, it's probably going to be a benefit to have an ATF representative working with the other federal agents that are already there," said Robert Thomas, an ATF director.
But some crime experts think all the police in the world won't help root out the real problem at hand.
"We've been having a problem with smuggled guns for years," said John Thomson, of the think tank the MacKenzie Institute. "There's a lot of police tasks forces on it and there's a lot of cooperation and it really hasn't put a dent on the market."
"The only way you're could really make a dent in the underground economy is by arresting people involved in it - the Americans."