I've heard this issue raised, and I've never understood why it's raised as a reason to weaken the arms rights of the citizenry. If anything, it is a reason to expand them and weaken the arms of the government.
Having said that, having greater arms often doesn't dictate outcome. We didn't fail in Afghanistan, Iraq, Vietnam, and Korea because of a lack of firepower. We wildly outgunned every one of those places and the people we were fighting. We failed because of a lack of will and a lack of patience. They were willing to do whatever it took to survive, and we weren't willing to do whatever it took to win. There was a level of brutality we weren't willing to inflict. There was a price we weren't willing to pay. There was a number of troops we weren't working to deploy. Furthermore, our enemies knew it.
Now put that same issue in place against American civilians. Yes, the US government in theory could crush any rebellion just like it could topple the North Korean regime (after nuking the place, deploying a million troops, and basically committing genocide on the North Korean people). It can deploy drones, nuclear weapons, etc. It can take machine guns to old men, women, and children and stop anything, but would it have the will to use the firepower that it has? I don't think it would. There are legal limitations on it, and even if there weren't, when the order is issued to just blow away a bunch of American citizens because your commander in chief doesn't like their politics, how many will actually carry it out? Considering that the military is disproportionately southern, I think many would not.