I'm sorry this got so long, but I don't want to rewrite the entire thing now...

> The evidence that these guns existed is scant

Yes, but the idea of them existing was enough to interest Congress, who tried to order 100 of them. They backed out of their order due to the cost. If they had existed at a reasonable price, one can assume Congress would have gone through with their order.

> The world of the Founding Fathers in 1788 was a world very similar to the world of 1688 and 1588

You're discounting the technological advancement of that era just because we've had more inventions in recent history. Innovation happened slower back then, but there were still large improvements in technology during the Founding Fathers' lives. Ben Franklin, who was one of the Founding Fathers himself, invented a lot of stuff during his life.

> You are looking at this with 21st century eyes

So are you. One of the reasons American citizens haven't had to bear their arms (some exceptions[1][2][3][4]) is because we have them and are able to use them if necessary. Few people are willing to die for a cause, and far fewer are willing if they have reason to expect their death will be in vain, and for their side to lose.

People are only willing to give up the rights they have now because they don't know what it was like without them.

> I am pretty sure that if you walked into the Constitution Convention of 1787 and laid out an AR15 and said, "One day a young man with this weapon will walk into a school and kill 20 children and 6 staff members with one of these and mass shootings happen with regularity." I am pretty sure the 2nd Amendment would be a lot different and a lot more clear.

We definitely agree that the Founding Fathers would be horrified by modern society. However, I don't think their reaction would be to limit firearm ownership. (Probably, they would probably want to know why tax dollars are going to the federal gov't to create public schools and why adults failed the kid) Would they have also changed the First Amendment to limit religion to Christian/Puritan variants? Probably not; they created the Bill of Rights to acknowledge rights that no human can morally take from another person, since the rights come from God*.

Ultimately, we can't prevent others from causing harm. Making black powder is easy and cheap, and tossing some fireworks in a pressure cooker or a pipe is something anyone can do. If we could hypothetically remove all guns from the entire world, badguys would cause harm through other methods, such as bombings or vehicle attacks[5]. We need to fix the root causes of these evils, not ban the weapon(s) they currently use so we can feel good about doing something.

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Lastly, I'm assuming you made this mistake in good faith but implying that our priority for minimizing mass murders should be banning AR-15 style rifles is misleading. Most mass shootings are committed with handguns[6]. So why not ban handguns then? Because in America, guns are used in a defensive manner far more often than they're used to murder people[7][8].

[1]: https://en.wikipedia.org/wiki/Battle_of_Athens_%281946%29

[2]: https://en.wikipedia.org/wiki/Ruby_Ridge

[3]: https://en.wikipedia.org/wiki/Bundy_standoff (not an endorsement, but it's relevant)

[4]: https://en.wikipedia.org/wiki/1992_Los_Angeles_riots#Korean_...

[5]: https://en.wikipedia.org/wiki/2016_Nice_truck_attack

[6]: https://www.washingtonpost.com/news/local/wp/2016/06/16/why-...

[7]: https://en.wikipedia.org/wiki/Defensive_gun_use

[8]: https://ucr.fbi.gov/crime-in-the-u.s/2018/crime-in-the-u.s.-...