It's very difficult to believe that lack of access to guns by the population in Canada plays no part in their lower murder rate.
Regarding your point that the figre I posted earlier for DC does not take the metro area into account, here's a listing of the murder rates for 300 major US metro areas.
http://www.crimetrends.com/id5.html
The DC area (I live in the District, BTW), still comes in at 7.4 murders per 100,000, compared to Toronto's 2/100K.
There isn't all that much different between the two areas- they are both around the same size, both areas have significant minority populations etc. So, what accounts for the differences in murder rates?
Fairfax County is not the whole area, just the largest jurisdiction in the area.
We have guns coming out the wazoo here, and our gun death rate is right down there with Toronto!
Montgomery county, PG county, DC, Howard and Charles Counties have HIGHER gun death rates than we do and they have extensive firearm restrictions.
That's why I am suggesting the factor driving the gun death rate, or the murder rate in general, is not the availability of guns.
We got'em; DC doesn't. Who has the lower rate?
Toronto doesn't have 'em; we have 'em. Our rate is usually right close to theirs. Of Toronto armed all of its citizens their gun death rate would probably not change at all. Then they'd be like Fairfax County.
Else, they'd be Republicans.