Where most of the population is unarmed and untrained, a relatively small junta can make considerable progress by not playing by the rules. That approach doesn’t work as well if every segment of the population is capable to self-defense. Of course, it is possible for one group to overmatch another, but the amount of logistical organization and preparation necessary for such a feat would also enable them to win elections…at which point victory by force becomes unnecessary. In effect, elections can be viewed as non-violent proxies for civil wars. The same is true of personal safety: in some countries, dissidents and merely insufficiently enthusiastic supporters of the ruling party could be rounded up at little cost to the government. In the US, every individual may be presumed armed and unpredictable, increasing the required manpower and decreasing the ability to arrest political opponents without repercussions.
So guns and other weapons in private hands keep certain political optimists from trying their luck at establishing the Second Caliphate or building Worker’s Paradise or Restoring the Republic. They end up trying to make it happen by means other than bloody mayhem. I’d rather have a PR battle and put up with annoying campaigns than have DFL and GOP (and possible other contenders) slug it out Beirut 1982 style. The Lebanese civil war, by the way, was sparked by outside influences that gave some factions the illusion of “short victorious war” being possible. The American civilian arms basically make violent political shortcuts backfire on those who try. That is why marksmanship and access to arms are important.