300M firearms, but a couple things to pay attention to. How many people own those guns, If a person has a collection of 500 guns, but only he and his wife will use them, than the # that matters is 2, not 500. Also, how many of those 300M guns are usable in this type of situation, how many would be brought to battle. So, is a 22 pistol going to be used in this type of movement? What about old guns from the civil war? What about guns that have not been maintained or were shitty to begin with, that can't be trusted. And lastly, firearms are only one component, the other is ammo. Sure, some preppers have 10,000 rounds, but most people have a limited supply. And you can bet that the first thing the govt would do would be to take control of ammo manufacturing and pipelines. It's not like the old black powder days when you could easily just make up a suitable round.
Still, I agree the number of gun owners with guns and with substantial ammo is enough to put up a fight, but considering the limited firepower, no tanks, no planes, no specialty weapons.... I think the fight would be heavily in the favor of the Military.
The one possible caveat to that is I believe much of our military would refuse to pick up arms against it's populous, especially on a massive scale.