You are imagining the conflict very incorrectly, imo.
If the U.S. government were to become "tyrannical" (by whatever definition we want to use to define tyrannical) they would be oppressing some on behalf of, or to the benefit of others. So it wouldn't be the U.S. government against all citizens, using their full military might to keep us all oppressed.
I've said it before, but my experience in the U.S. Navy is that the military would not simply oppress the masses. In fact, it is illegal for the U.S. military to perform any domestic police actions. Also, all members of the U.S. military swear an oath to defend the U.S. constitution, not the president, not the government.
So, anyway. Imagine civilians, some opposed to this new tyrannical government, some indifferent and some who support it. The antis don't all live in the same town. The government can't just bomb out residential neighborhoods for good measure.
The right to bear arms does not mean local bands of militia fighters meet the U.S. military on the battlefield and stand their ground. It does, however, mean that to oppress certain groups to the point that they have nothing to lose means that when government agents try to enforce whatever oppressive measures against them will have an extremely arduous and risky task.
It raises the price of tyranny, hopefully to a level high enough that it isn't worth it.
It also draws a definitive line. If the government will use military force (which I don't think is really possible since U.S. soldiers wouldn't carry out such orders) against civilians it will make it harder for the public at large to be indifferent or to support the government.
So, anyway, lyfao all you want. You are not considering realistic scenarios when you soo gleefully chop down your straw man.
Just wanted to point something out on second reading...this ^ is not a pro-gun argument, like at all.