> How do you see that happening? What is the reasoning?

There are numerous stories throughout history wherein firearms were used to protect against oppression. In some cases, those firearms were used against governments, nation-states, guerrilla forces, insurgents, etc. In other cases, they have failed. Regardless, the odds are dramatically skewed when one side has guns and the others don't.

Reductio ad Hitlerum, before "Germany" became "Nazi Germany", there was far less in the way of gun control. The regime prior to Hitler had made gun control a priority, so the citizen was generally not armed. Before Hitler began committing genocide against Jews, he actually loosened gun control regulations... for everybody that wasn't Jewish. I'll spare you the remainder of the details, with which I'm sure you're at least somewhat familiar.

Before Hitler, there was a Revolution in the 1700s from some would-be sovereigns that claimed Independence from Britain. Before that revolution, the redcoats attempted to seize the arms that would later become the implements of war by the militia. In some cases they were successful. In either case, Americans found that they hadn't been armed enough, and later had to entreat with France for more arms to be provided. Long story short, then America was born.

These are not particularly unique stories. There are many stories where governments have committed genocide en masse. There are less stories where those governments enacted gun control and disarmed citizens prior to committing that genocide. There are fewer still stories where those citizens were not disarmed, and of those, there are stories where the citizenry was able to escape from, defray, deter, or even prevent those catastrophes from occurring.

> doesn't seem unreasonable that for guns to be a viable threat or worry-some thing for the government to fear

Quite probably, yes. That doesn't mean that the gun will be the only implement. It's probably even more so now, but let's not forget how out-gunned we were compared to Britain. But despite the improbability, we won that. It's also worth noting that you might not be fighting against the federal government, but perhaps a local government, or rogue police officers. The "Battle of Athens"[1] occurred in the 40s, and was fought against a corrupt county level government.

> And again there having or not having legal guns seems to make little difference. What am I missing?

That the objective need not necessarily be "to defeat the US military". A successful outcome might be to defend against slaughter just long enough to escape, as some Jews did in Germany that had hidden their firearms from confiscation. A successful outcome might be to simply highlight that the US government is waging war to the general populace. There are a variety of successful outcomes between "die" and "successfully defeat the entire armed forces with my shotgun" that could be attained.

An all-out war between the federal government and the US citizenry is definitely a lop-sided battle. Drones operate out of range of being shot down, and even the assaultiest assault weapon does little damage against a tank or armored vehicle. That said, that's the least likely type of skirmish to occur. For 1) the US government isn't likely to declare any such type of war, nor could they easily justify it, even with media control. What is more likely to happen is something like the Japanese Internment, or a WACO style of incident. Even in those cases, our odds are slim of obtaining an outright 'win', but holding off attackers long enough for the media to catch on that the government is trying to kill you might be? Etc.

[1] - http://www.constitution.org/mil/tn/batathen.htm