The 2nd amendment made sense when the enemy was a group of people across the ocean (the British), and other groups of people from across the ocean that they hired as mercenaries (the Hessians). Keeping the common people armed to be used as foot-soldiers in case of foreign invasion is not a bad idea if invasion is a serious concern; Switzerland still does this to this day.

For protecting against your own government, it really doesn't make that much sense. Your own government has to have support of your own military, which gets its members from the population: the military is made of your own neighbors. If your military is committed to the government and doesn't mind shooting their own family and neighbors, then you have a problem that arming people with small arms isn't going to solve: the rebels just aren't going to be that numerous. More likely, the military isn't going to support this action at all, and will mutiny and either implode as different factions within the military fight each other, or the military will stage a coup and take over the government (this has happened before many times, in other nations). In short, if the military supports the dictator, the armed opposition really has no chance of winning. If the opposition has a chance of winning, they don't need weapons because the military isn't going to support the dictator.