Quora has considered this question and this is the highest upvoted response.
What would happen if America "accidentally" nuked Moscow?
I wouldn't even think of such a thing.
Because by the time the nuclear bomb entered the airspace right being blown out of its missile silo, countermeasures have already been made by the Russians. They are not as weak as Japan when it comes to countermissile technologies.
The question, if it ever happen, is an insult to the military capabilities of this state. Russia, although unlike the US in military might, is still capable of prolonging an impending nuclear war, at least more than two months. Their techs, though obsolete and ill-maintained, can still shoot down US hardware.
And, by the time the nuclear bomb is about to enter Moscow airspace, all government dignitaries have already been evacuated. Continuity of government is ensured. Russia is a city of secrets, so we, as foreign observers, can never really describe perfectly their anti-nuclear bomb emergency evacuation systems.
For those who say Russia will be nuking the US too, that's a big mistake. Russia, knowing its capabilities both economically and militarily, will not rush to declare war, because it will be overwhelmed, and will lose such war. We don't underestimate the former communist nation, but that's a well-calculated reality.
Russia needs to stay on top while the US loses its power to influence the world.
Putin the Strategist knew that the only way the world will fully trust Russia (and China) is by painting the US as the Imperialist War Lord, the Killer of Civilization, the War Hungry Nation, the Whatever-Bad-Thing.
And the largest evidence to support that is for the world to see a picture of a nuke-torn, nuke-destroyed Moscow.