I have already explained it in previous posts.
Well...I already responded to your point in my previous posts, but that doesn't seem to have stopped you. Anyway, the only "explanation" I see is your insistence on the greater force that a tyrannical government can bring to bear, which doesn't convey any more content than saying it's a myth that guns will help citizens in the event a tyrannical government wants to squash them. That is, there are two propositions here:
1. It's a myth that guns will help citizens in the event a tyrannical government wants to squash them
2. Tyrannical governments can bring to bear overwhelming force against citizens armed with mere semi-auto/automatic weapons
Neither of these explains the other--someone who believes 1 almost certainly believes 2, and vice versa. What you've really done, instead of explaining, is just re-stated your point. Again, I've already responded to that point, in two ways:
1. Actually, it's false, as contemporary examples show: Afganistan and Iraq. Previously, Vietnam was another good example. Guerilla warfare and asymmetric methods work.
2. In any case, if your argument works at all, it works as an argument for why private citizens ought to be much better armed than they are at present--why it ought to be legal for me to go purchase a tank or an Apache helicopter or something, and why the government ought to subsidize such purchases by private citizens.
And now let me add yet a third response:
3. Furthermore, the question of who would win a pitched battle is not the only relevant question. Pursuant to my point about guerilla warfare, the question is not just whether if we lined up private citizens on one side with their semi-auto/auto rifles, and the military with all its artillery, missiles, armor, air support, and so on, on the other, who would win. If a tyrannical government (and it's not just government. Corporations, or any powerful entity, are potential threats here) did decide to enslave citizens, it has to calculate the costs of doing so against asymmetric methods. It has to calculate how likely it is that citizens armed to a certain level might be able to capture heavier weapons. It has to calculate the odds that private citizens know the local landscape well enough to be able to harass an army repeatedly, and then melt away into the hills. And that calculation becomes a lot easier when the citizenry is unarmed, or virtually unarmed. A group of ten guys with some survival training, lots of ammo, knowledge of the local terrain, and time to prepare, stand a very good chance against a hundred infantrymen without those advantages, provided those ten guys can lay down quick pinpoint fire into enfilade positions from concealed defilade positions. You can do that with semi-auto or automatic weapons. You cannot with bolt action or single shot rifles.
In short, quite a large number of considerations play in here, and that's the whole basis for asymmetric warfare. Your chances go up if you and your buddies are armed with AR-15s with semi-auto and burst-fire options. Your chances go down if you and your buddies have to work a bolt to fire a single shot--and those two points are true regardless of how the enemy is armed.
