The underlying question is. "Did the founding fathers have any idea that arms of any sort, owned by the average person, could be this deadly?".
This argument completely misses the point of us being able to defend our freedoms against government tyranny.
Because of this point alone this argument simply doesn't hold water.
It can however be used to teach about the true story of our nations founding, and foster Patriotism, which IMHO is huge help to us rather than them.
That being said, the arms that they used at the time were in my trained opinion deadlier than what we have now.
The best visual example I can think of is the movie "The Patriot". The battle scenes were pretty graphic, and as far as I can see accurate.
Think about those battle scenes for a minute with this in mind, pretty much anybody with any significant injury stood a high chance of death from sepsis (infection, usually bacterial).
Losing a limb, (sawed off while you were awake and screaming) but surviving through the inevitable infection, was a success.
A bullet wound that killed you quickly could have been viewed as a blessing, these guys were hacking each other to death with edged weapons in bloody battles.
Have you ever seen anyone die from infection that was introduced through severe body trauma? I have seen too many in my career in nursing. Think nearly unending agony.
Trust me you'd rather be shot dead where you stand.
Nevertheless, dead is dead.
I can guarantee you that the Framers of our Constitution attended a long list of funerals of friends, comrades, and loved ones for months after each battle.
The whole argument is Hogwash and is patently insulting to memory of what they all went through. The person who brings this up to you should be treated to a history lesson of they are sincerely looking for answers, and treated with severe contempt if they are using this for political gain.
Consider yourself armed with truth to win this particular fight.