The actual argument is that our weapons are much more deadly than theirs were. This is a complete and utter falsehood.
What they are not taking in to account is that Medicine has evolved as well. A bullet wound today is not nearly as likely to kill as it was back then. Period.
Swords, knives, pikes etc. were far more deadly back then than any semi auto rifle is today. The death it led to was for more gruesome and painful. A metally deranged person in the 1780's armed with a brace of pistols, a musket, and a sword could easily have killed MORE than this Mentally deranged person.
These men (the Founders) almost without exception participated in the fighting at some point. They were quite aware of what weapons were capable of doing, they saw the consequences close up and personal. They got to watch their friends die slowly and horribly from wounds that we could deal with in minutes now.
DO NOT LET PEOPLE GET AWAY WITH TELLING THIS COMPLETE AND UTTER LIE!
Gaily bedight, A gallant knight In sunshine and in shadow, Had journeyed long, Singing a song, In search of El Dorado