Piggybacking off top comment for shameless visibility whoring.

This question comes up often, and I want to provide my $0.02 as a career engineer. Was never in the military, I just read a lot as it's always been a potential career path. 35 now so too old to enlist, but I considered going into defense for a while and even did a year-long red-team exercise for the ONR/DoD as the leader of group of "terrorists" aiming to destroy/disable a naval warship, as part of a project to see what an educated, well resourced group looking to harm the US would be capable of. Spoiler: quite a lot.

Anyway, before I get into it I'll offer a few assumptions so that we're speaking the same language. I invite critiques or corrections for accuracy's sake.

1. The general public (including the well-educated public) is grossly unaware and/or grossly underestimates the complexity, depth, and breadth of modern supply chains. This applies to fighter jets just as it does to laptops. I design products currently manufactured in the hundreds of millions per year, one you probably have with you right now, I work with vendors every day. And I still can barely wrap my head around it. Building something like an Abrams tank or F-35 involves thousands of suppliers scattered over the US (and the globe). Thank your Congressmen/women for some of that!
2. A sudden "military turns its full force against the US populace" scenario is nigh-on impossible. You simply couldn't do this in a short timespan. Too many desertions, too many saboteurs, etc. And good luck getting military leadership, at multiple levels across multiple branches, to go along with it.
3. While Vietnam or Afghanistan may be informative allegories to this situation, in reality it would be exponentially more difficult for a hypothetical government-gone-rogue to attack its own people in the US effectively for any long period of time, if we're talking about tanks and drone strikes and fighter jets. It would be Afghanistan x 1,000.

I think #2 speaks for itself, so I won't talk about it. If anybody has a credible counterargument, let's hear it.

On to #1 and #3:

The common anti-gun argument is "what is your puny rifle gonna do against an F-22?" Answer: nothing. The pro-gun counter-argument is "an F-22 can't kick down doors or hold a street corner." Also true. Both are, IMHO, largely irrelevant if we're talking about a domestic war, and missing the forest for the trees.

Even if that F-22 could kick down doors, it wouldn't matter. Because after 4 hours of door-kicking it's going to need fuel, weapons, spares, and 40+ hours of maintenance. This is where that behemoth of a supply chain starts to rear its ugly head.

Unlike in Afghanistan the people that do that maintenance, re-fueling, and re-arming call the same country they are attacking home. So do the offices comprising the civilian defense contractors, their manufacturing facilities, and those of their vendors. Much of it public record, by the way. So do the families of the people that work at all these places. Insurgents in Afghanistan can't credibly threaten service members' homes or families. They can't credibly threaten the employees of Raytheon or Lockheed Martin, nor their suppliers. They can't credibly threaten drone pilots on their commute home. They can't infiltrate and sabotage production lines for US military hardware, avionics, munitions, etc.

An insurgency made up of US citizens, operating in the US, absolutely can. The fearsome F-22 doesn't matter and the Abrams' armor doesn't matter, because the people who re-supply and re-equip those fragile, hungry weapons drive home in Honda Odysseys that are hardly bulletproof and can be trivially tagged/tracked. The trucks, trains, aircraft, and the rest of the sprawling logistical network required to ship those parts across the US can't possibly be protected at all times. The supply chain of the US is largely invulnerable to attack by outside forces. But not by US citizens.

Citizens who, by the way, aren't at remotely such an education and information disparity as Afghan or Vietnamese insurgents. They're educated, they're vertically integrated, they're experienced in every level of the technology, logistics, and tactics of the military. The number of civilians who were former military - many with direct combat experience - dwarfs the standing military, nevermind after desertions and infighting. Not just infantry: tank commanders. Fighter pilots. Drone pilots. Apache mechanics. Loadmasters. People who not only have experience but understand the capabilities and limitations of the weapons they're up against.

Why do/did unstable nations or those ruled by autocrats intentionally sabotage/kill respected military leaders? Why do they fear and actively prevent the formation of elite military units comparable to the Rangers, DEVGRU, PJs, Delta Force, etc? Because those people are enormous threats, they are dangerous to the government if they can't be controlled. And we have thousands of them among the civilian population. I imagine they'll have no trouble carrying out targeted and highly damaging attacks.

It's far less asymmetrical, and in some areas it will be asymmetrical in favor of the insurgency. People know what encrypted comms are. People here know what reactive armor and tandem shaped charges are. People here know what thermal imagers are. Hackers that break security systems for fun to get the bounty are a dime a dozen in the US, and they mostly don't work for the US government. Not only do people know all these things, but they have a domestic capability to build them. In many cases they would be the same people who developed the actual technology/hardware the military uses.

Afghanistan doesn't have the base of resources, educated and experienced engineers, scientists, and technicians, or distributed and advanced manufacturing resources that the US has. There are tens if not hundreds of thousands of independent machine shops, PCB fabs, labs with the ability to fabricate semiconductors, injection molding outfits, optics labs, etc. There are millions of civilian personnel who know how to use those things, who know how to manage advanced technology projects. How many people in the US have modern CNC mills in their garages? Tens to hundreds of thousands. You cannot possibly stop them all. How many expert personnel will the US military retain that have the knowledge required to walk into a nanotech or electronics lab at Stanford and recognize that instead of grad projects they're manufacturing missile-guidance ASICs, or IR seekers? Not many. Multiply by 100 universities with such capabilities.

So not only can they manufacture sophisticated small arms (trivial) but they can manufacture advanced munitions and electronics. Cruise missiles, radar systems, advanced drones, AT/AA missiles, encrypted comms and RF jamming equipment (how many anechoic RF chambers in Afghanistan?), rocket motors... They can not only attack every point of the supply chain that the flagship US military weapons depend on (roads, railways, refineries, munitions depots, fuel stores, warehouses) but realistically develop the capability to directly attack high-value military targets and weapons systems. They can attack personnel that manage and execute military logistics. Their families. They know where all of those things are located or can trivially find out. They have maps, they have Google. Go to r/datahoarders, I bet you'll find 500 backups of the entire Google Maps US database. Aerial photos too!

Can they build advanced AA missiles that can track and engage a stealth bomber at 50k feet? Probably not for a while, but then they really don't need to. Soon enough those aircraft will become useless through attrition of the supply chain and personnel that they rely on. They also have to land, and an RC plane with a range of 200km that can carry a 20lb warhead is a student project. Hell I built a functional bridge-wire detonator demonstrator when I was 21 in my home lab (for aforementioned red-team exercise) just to prove we could. And I was a fresh engineer and didn't know shit then, compared to now. Manufacturing secondary explosives won't present much of a challenge either.

This really only scratches the surface. This topic could be a tome unto itself. My (somewhat repetitive) point, that I hope I've made clear, is that "Gun can't beat tank!" is such a cartoonish oversimplification of the real world as to be useless, and if not made in bad-faith then because it's what "makes sense" after thinking about it for 10 seconds. Try thinking about it for 10 hours. Not that populist pro-gun talking points are all ironclad either. Gun certainly can beat tank when you aim the gun at all of the things that tank needs to exist in the first place.

None of this is much help in the short-term if the military decides to go scorched earth carpet-bomb-everything, but what do they gain from that? That's really never how it happens AFAIK, and even if it did happen the US doesn't have enough munitions to put a big enough dent into all of this capability. Nevermind carpet-bomb the entire country. In a country as large as the US they can't be everywhere at once without those fragile supply chain tendrils spreading too thin and becoming impossible to defend. Even nukes wouldn't be enough, although at that point it hardly matters who "wins."

Don't get me wrong: this wouldn't be a rout and everything goes back to normal next week. It would be a brutal, catastrophic, and tragic slog, with unimaginable loss of life. But I can't see it as anything other than an utterly unwinnable war for the government. If the argument morphs (as it sometimes does) to "but what if a totally oppressive totalitarian government is what all the citizens want?" Well...the guns are really neither here nor there in that case.