Other countries have less guns and possibly fewer gun deaths, but that does not make them safer. According to Jeffrey Miron of Boston University: Countries with the strictest gun-control laws also tended to have the highest homicide rates. Also according to the U.N., as of 2005, Scotland was the most violent country in the developed world, with people three times more likely to be assaulted than in America. Violent crime there has doubled over the last 20 years. 3% of Scots had been victims of assault compared with 1.2% in America. In fact the major surveys completed in the past 20 years or more provides no evidence of any relationship between the total number of legally held firearms in society and the rate of armed crime. Nor is there a relationship between the severity of controls imposed in various countries or the mass of bureaucracy involved with many control systems with the apparent ease of access to firearms by criminals and terrorists.”
As for the Founding Fathers the notion that they only expected people to bear smooth bore muskets is also false. Smooth bore may have been the primary weapon in the Revolution in the 1770’s, but by the time the 2nd Amendment was written about 1789 rifling was seen, as well as multi-shot weapons. There is no evidence or logical conclusion that they would have expected weapons to not advance, or that they would not want ammunition to be bought as well. As you said, the Founding Fathers were very smart men. I don’t see where they made a mistake in this regard.