If it breaks out as a succession, it very well could end with one side winning. That isn't what we're discussing, though. With a succession you have 2 standing armies slugging it out. The US Army fighting citizens with ARs wouldn't be the battle. You'd have, to use the same example, the northern army fighting the southern army and both sides would have the modern tech and capabilities that came with their territories.
An insurrection, where the discussion of armed civilians comes more into play and what this discussion was about when I entered, is people from the block going against the government. To use your example, what do you think would happen if the US Government nuked a domestic city? What do you think the average military member's reaction would be? What would happen if the US used their advanced firepower in Miami? What percentage of Soldiers called to carry out that strike would participate? Would it endear the locals and state to the government? Would it cause locals to back down, or join the cause? Would escalation cause more Soldiers to turn their guns on their neighbors, or back down?
It doesn't matter how good the Army's firepower is if Soldiers won't wield it and if wielding it turns more people against the government. Soldiers see themselves as protectors of freedom and America. They don't seem themselves as the armed thugs of politicians.
Like I said, this gets chess boarded at high levels regularly. It's not a great outcome for politicians. Heck, we saw what would happen in many places on January 6th. Police/Soldiers opening the doors and welcoming them in.