That's not really how it happened. Small groups with guns is not what did anything about Assad's forces (and in fact that was partly what caused chaos is those groups got usurped by extremist groups and everything devolved into chaos). They didn't make any inroads until we started getting serious about bombing, providing anti-tank weapons, military training, and other supplies, and even then they basically only made progress because of Assad's military having a lot of defections and having their power held in check (as in if they'd continued rampant use of chemical weapons we would have started bombing the hell out of Assad and his forces like how we went after Ghaddafi and Saddam Hussein). That's even what triggered Russia getting involved is Assad was starting to weaken and they knew he'd get desperate and then would probably resort to whatever he could which would then trigger the U.S. and NATO to take his regime out using direct military means. Plus there's those millions of people who fled Syria so Assad's forces simply had less people to brutalize. There's a couple of good reasons why Assad didn't go after ISIS, and one of them is because ISIS, who has a lot of military hardware they got from conquest in Iraq, would have put up enough of a fight that his regime could have lost to them.
Small arms accomplished little to nothing in the overall Syrian conflict. Same with Afghanistan when the USSR invaded. They were getting their asses kicked (machine guns weren't terribly effective against tanks and Hind-Ds) until we started providing missiles. And our military is much stronger against missiles (and IEDs now too) than Assad and the 80s Soviet Union hardware.
The U.S. military would bitchslap the everloving fuck out of these militia groups if they had any actual intention of oppressing and quelling them. Even if they had fully automatic weapons and very probably even stuff like anti-tank/anti-helicopter missiles. They wouldn't even need to put boots on the ground to do it either. Effectively the only way those groups would realistically have a chance is if they resorted to ISIS tactics in taking over civilian population centers and effectively taking people hostage and the military not willing to suffer the collateral damage to eradicate them. At that point, those groups would be outright terrorists, so any revolution they were wanting to achieve would be lost as they attempt to exert their own form of oppression (they're not going to just let the people flee, as that will leave just them there, and then the U.S. could justify the casualties and would wipe them out, they're sure as shit not going to let them open up shop and function as a separate entity on American soil).
Our best defense against a tyrannical government is knowledge and understanding and preventing it from occurring. I get that people think gun rights is key to that, but I frankly don't. Not at the individual level. I absolutely take issue with various government things (NSA's overreach, some of the law enforcement things they've been doing via the FBI, police corruption) that absolutely is in the form of oppression, but I also know that having a gun will do nothing about those things and the best thing is to try to bring calm rational discussion and work to get the bought off crony politicians out of office and some of the, in my opinion, borderline insane people from some of the agencies that could dictate things (CIA, FBI, NSA). Cut back on the rhetoric and fear, get serious about making meaningful reforms.
The other key thing is for our states to maintain their militias that use real military units and hardware. That is the well regulated militia intended with the 2nd Amendment.
However I'd say the most important thing is for us to prevent a division like that from occurring in the first place. We've been through worse stuff than what people are so contentious and divisive about these days.