There is much more involved in resisting tyranny than just everybody possessing an AR-15 or equivalent.
Those with military experience would be quick to point out that "an army without organisation and discipline is nothing more than an armed mob."
Much of the early successes in the American Revolutionary War came about because the American militias were often better trained and better disciplined than the British troops who were sent against them. Many of the British soldiers were little more than raw recruits, with little training and no experience of warfare. (That was particularly evident at Concorde and Lexington).
The only situations in which a disorganised mob could overthrow a government are when the military either mutinies, or (as happened in the Iranian Revolution in 1978-1979) stays confined to barracks. Furthermore, as other posters have pointed out, whether that disorganised mob was armed or not would be of little consequence.
To suggest that Hitler's successful mobile "Blitzkrieg" attacks of 1940 - involving careful coordination of tanks, aircraft, artillery and motorised infantry - could have been turned back by mobs French and Belgian citizens with small arms is surely stretching credibility?
On the other hand, if one was looking for a medal for getting themselves shot, then I guess that would have been a good way to go about it!
The only way an armed citizenry could successfully resist either tyranny or invasion is if they were part of a trained and properly organised militia (as in the case of Switzerland).