The weakest argument for gun ownership in the USA

[Kralizec]

What are the chances US citizens will overthrow the federal government with machine guns bought in the local gun shop? Close to zero too. Still, there is no law saying it is illegal to have pink elephants in San Francisco.

The chance of a militia in say, Ohio, capturing Washington DC and thus overthrowing the government there would be slim at best.

The chance of a seceding state protecting its autonomy is much greater, especially if other states would follow its example.
I know you're thinking about the Confederacy now, but you have to realize that it comprised less than a third of the total population and the Union's success depended on the willingness of the population to fight a rebellion they viewed as illegitimate. If the US government were to become truly tyrannical it's questionable wether people would agree to be drafted to supress rebels.