Prime minister-designate Justin Trudeau's campaign promise to sign the United Nations Arms Trade Treaty will take a lot more work now that Canada has missed the official deadline to join.

The Arms Trade Treaty is an international deal to regulate the global trade of conventional weapons, everything from guns and ammunition to tanks and fighter jets.

So far, 130 nations have signed and 77 have formally ratified the deal. But Canada remains an outlier, the only member of NATO that hasn't joined.

Despite repeated assurances by experts that the treaty would not affect gun owners in Canada, the Conservative government refused to sign.

"If properly done, an Arms Trade Treaty can help limit the worldwide trade in illicit arms," said a spokesperson for then foreign affairs minister John Baird in September, 2013. "At the same time, it is important that such a treaty not affect lawful and responsible firearms owners nor discourage the transfer of firearms for recreational uses."

But now that the treaty is in effect, the process to join has changed.

"States who have not signed the treaty by Dec. 24, 2014, do not sign and ratify the treaty but rather accede to the treaty. In order to accede to the treaty, Canada must ensure that it has put in place domestically all legislation or regulations that would be required to ensure that we can fully meet the obligations of the treaty," the Department of Foreign Affairs said to CBC News in an email.

Annual reports required

Ken Epps is an expert on the arms treaty and works with Project Ploughshares, a peace group in Waterloo, Ont., that monitors arms exports.

He says there are a number of things Canada must do to comply with the treaty. "Currently there is a free-trade situation between Canada and the United States, which means that military goods that are exported from Canada to the U.S. are completely unregulated. There is no export permit process required."

The government would also have to regulate arms brokers, which Epps says would be in keeping with several other treaties and commitments Canada has made.

Then there is the requirement for detailed annual reports on military exports. Canada has produced these reports sporadically since 1990 and Epps says they lack critical details.

"Armoured vehicles that are being sent to Saudi Arabia are listed as ground vehicles, and that is the full extent of the description, and there's no detail on what kinds of weapons systems have been added to those."

Epps says that kind of information will be important in the future for determining whether Canada is meeting its commitments under the treaty to conduct risk assessments before transfers are made and authorized.

In his opinion, Epps doesn't think the controversial $15-billion deal to supply Saudi Arabia with armoured vehicles would have gone through had Canada signed the treaty.

'Unusual optic for Canada'

Canada's refusal to join the treaty earlier has had other effects, according to Sarah Parker, a senior researcher with the Small Arms Survey in Geneva.

She says those who signed, even if they have not yet ratified the deal, have been included in important decisions about standards, implementation and the role, location and mandate of the treaty secretariat. "Canada was very much missed from the negotiation process," says Parker.

"We started to see the sort of language that usually we expect to see from the United States in terms of the focus on civilian possession and an anxiety about efforts to control guns at the domestic level through international regulations and treaties such as the ATT.… The U.S. has signed, of course, and Canada hasn't, and that's, I think, an unusual optic for Canada."

That said, Parker enthusiastically welcomes Canada's eventual accession to the treaty. She says there are still many big decisions to be taken and many more steps toward implementation where Canada can play a very important role.