"I think it was probably an oversight at the time with everything happening so quickly," he said, adding that police would learn from the mistake.
"[We] need to be more conscious of that in the future where large groups of people, particularly young people, are at risk."
Ferguson didn't know how the gun was stolen, but said people could sometimes be "careless or irresponsible" with how they secured their firearms, although "despite the best security, some of our villains are determined and will find a way to break the firearms out of their cabinets".
"I think it's a topical subject at the moment because of what the United States are experiencing.
"Firearms in the wrong hands are lethal weapons and we need to make sure that the privilege of having a firearm is [taken] seriously are all precautions are taken to prevent them falling into the hands of the wrong people."
The man has gang links and faces charges on other matters, Ferguson said.
There is no indication what the gun was being taken for.
The man has been remanded in custody until a court appearance next month.