"The AR-15 and its analogs, along with large capacity magazines,
are simply not weapons within the original meaning of the individual
constitutional right to 'bear arms,'"
This old red herring, trotted out once again...
The same logic also says that there is no protected speech if written on a typewriter, word processor, computer, internet, or any thing other than written on parchment with a quill pen and iron gall ink. And, so, therefore the government may censor or outright prohibit any speech using modern communications.
Is there ANY modern firearm/design now being manufactured for general use that did not get its start as a military weapon
I'd say that depends on how you define "get its start". And how specific you are about defining firearms designs. There are lots of designs that were made to sell to any customers, and the military is a big potential customer. Some designs caught on with the military right away, some took some time (and development) before the military(s) were interested. Some designs never did catch the military eye.
The military didn't buy many of Colt's first revolvers, but later, they bought a LOT of his following models. The first semi autos were ignored by the world's militaries, who later changed their minds as more suitable designs came along.
Tell me a Ruger No.1 got its start as a military weapon. It didn't. You can make a case for the general design of a falling block having been used as a military weapon in the past, but you can't claim the ruger as anything but a pure sporting arm.
The double barrel and pump action shotgun didn't get its start as a military weapon, and I'd venture to say neither did the semi auto.
And, even those arms which were designed for military use aren't anything to be ashamed of. They are our right.
And that's their entire argument, that we shouldn't have military arms, period. This was also the opinion of the Crown troops attempting to quell those rowdy colonials in Massachusetts. On April 19th, 1775, they tried to do something about it. It appears that what they failed to do, then, is being done now by a Massachusetts judge...