This. Even if they outnumbered, say, a platoon of Marines sent in to confiscate their guns, it would be a very short, very one-sided fight. The Marines are trained in group fighting tactics while the nutsos are islands in a storm. They'd be individually overwhelmed very swiftly. They're mostly too paranoid to work together.
1. How many times do the Americans have to lose Vietnam again and again all across the world from Iraq to Afghanistan before realising that the USA hasn't failed because it didn't apply
enough firepower, the US failed because you're not supposed to send in the Marines, you're supposed to send in the police, and the police aren't supposed to have more weapons than the marines. By the time you're treating a civil issue as a military one you've failed, game is over. The British failed to suppress the American rebellion because they were fighting fucking nutso islands in a hurricane, capturing one of their bases or units meant fuck all to the rest of the nutsos - the professional elite force will find victory after victory does absolutely nothing to actually destroy the enemy, like an oil slick spread over the ocean. Drone striking every American who refuses to hand over their armaments would not strike sufficient fear into the heart of the American people that they would surrender. Hell no. They'd be disgusted and fight harder. Looking at the British experience in Ireland or India, even when the force discrepency between a civilian populace and a post-WWII war machine was at its highest, every fight was politically insufferable for the British government, same here. You'd be handing the narrative on a silver platter with a very ostensible display of tyrannical power.
2. It wouldn't take much for the militias to organise in the same manner that the Ukrainian ones did, and everyone seems to be overlooking the obvious potential in millions of rednecks who do bugger all but shoot and fuck all day led & trained by any number of the USA's current population of 18 million+ veterans everyone forgets about, of which there's a significant portion of homeless, suicidal or sick. To paraphrase a British officer fighting undisciplined manchu infantry: they were not as well-trained, but manchu bullets killed just as easily as theirs. Whose to say how many of those 18 million would oppose surrendering their weapons? For whom would they fight?
3. Lone nutsos & group nutsos would not stand in the open waiting for the full weight of the US military machine to grind them down, especially in the likely event of their ranks including veterans. Civil wars hurt for a reason, everyone knows their enemy. Using an elite mobile force to patrol a sea of American towns populated by unknown quantities of nutsos presents the significant problem of your elite units being in the open, the nutsos in the crowd. Or innawds. They can't be everywhere at once, and they've also got to be sensitive of disrupting the public lest they incur protests & obstructions, or dissent within their ranks
4. We're assuming that no one within the armed forces would be sympathetic to the 2nd amendment, which seems like an alternate reality stipulation
5. Organised crime & foreign intervention will confound all US efforts at the first sign of weakness. A divided, well-armed populace would very easily find sponsors stoking the flames. If you believe foreign powers are willing to hack your elections and fuck up your infrastructure in peacetime the likelihood of them sitting out a perfect opportunity to permanently cripple your country is 0%. Attacks on US communications, intelligence, infrastructure, flooding the US with guns, drugs, cash and foreign volunteers. Already US domestic terrorism is trained internationally, a war would be a marsh of death
6. Your intelligence would be key. Considering the state of US intelligence, blighted by leaks, corruption and interservice rivalry... It's optimistic to say the least.
7. If you look at any of the uprisings of the 20th or 21st century from China to Eastern Europe to the Middle East, where rebellions often simply cannot win, it is enough to achieve a breakdown of the ruling regime's control simply by ensuring that the ruling regime also cannot win. Even with armoured regiments and air power, a tank and a jet cannot police a street or collect taxes against a belligerent population, which means that a lengthy occupation by ground forces would be necessary. If this is not possible across the vast geographic size of the USA, administration breaks down and local strongmen or security forces establish their own power bases, and gradually the central government's authority does a Libya. Which is to say it disintegrates and all the security forces or militias left standing at the end claim their slice of pie
8. You always leave a way out; sending in an elite military unit to conduct a police action is going to result in obvious undesirable outcomes. At best, people start burying guns up their arses, at worse they react like cornered rats.
9. Go for the pistols & ammunition. Handguns are the biggest troublemakers because no one walks around with a rifle in their pants, while upping the price of ammunition starves enthusiasts out of guns or money, whichever buckles first. Couple that with gun buybacks and effective border control to cut down on black market trade of guns, and you have a much better shot at successfully disarming the majority of the US population without incurring the wrath of #patriot Americans or harming Americans living in the middle of bumfuck nowhere who have legitimate uses for armaments. Keep it from ever becoming a military situation