I have no doubt there are weapons tons of weapons in Canada.
...but their disassembled and parts kept province wide, same with weapons, but we don't even have the full assembly in the province. all the weapons found(realistically) would be old black market pieces(if they were still around) and/or military pieces from any uprisings and riots and such, but there was a war going on. most of our troops(being US troops after the annexation{is that a word?}) would be on the frontlines or waiting at their stations, most likely nowhere near ontario.
the military weapons would number in the few, as they would only be attainable from outfitted soldiers, who would mostly be on the frontlines at the coasts or overseas. keeping large stockpiles of weapons disassembled is standard military practice so in case of riots in cities where these were stored civilians wouldn't have access to working military-grade weaponry.
yes, there would be soldiers stationed all across Canada in order to "keep the peace", but Canada is pretty big. I'm pretty sure were the second largest country, although most of our population is located along the border.
if the US was trying to hold an entire country the size of ours while at war with multiple other nations, there would be a huge displacement of troops, kind of like what happened to the old roman empire. the populace would easily overwhelm the military force, but with legal firearms, which probably wouldn't be as exciting (like Styles said, a lot of hunting shotguns/rifles, revolvers and semi-auto pistols).
very few people in canada actually own a firearm(legally, black market is completely different. that crap wouldn't last 200 years), haven't you guys seen "bowling for columbine"? the US has always been known as a country whose population invests heavily in firearms. I just don't think that because the US decided to put all their money into nuclear technology and military expansion Canada would be more violent and do the same.
all I was trying to say is it wouldn't be very realistic, and TBH, I just don't think Fallout will ever leave the US...they went all the way north to Alaska, but no Canada or even Mexico? c'mon.....