Our military has much, much more than simple AR-15s. In fact, their base M-16s are superior due to their full-auto capability, which civilian AR-15s do not have.
As far as what George Washington and the founding fathers envisioned, you underestimate them. Read "The Federalist Papers" for more on this.
By your logic, to put the first amendment in the same context, the laws governing freedom of the press are only valid for newspapers whose presses are hand-operated and use fixed lead type. After all, no one in 1776 foresaw offset printing or electricity, let alone radio, TV or satellite transmission.
As for the inappropriateness of rifles such as AR-15s and the like, tell that to the Korean grocers in L.A. during the Rodney King riots of April 1992 how such firearms are "not for the general public". When the local police told them "we're overwhelmed -- you're on your own", these citizens were able to adequately defend themselves against the hordes that were burning and looting everything in sight.
Do not mistake me -- if you wish to be defenseless in the face if such adversity, feel free. But do not seek to enforce that same level of inability on me.
Nope, don't think I'll be defenseless. I think I can handle self-defense just as well with a Mag-44 handgun, 9mm, 12-gauge if they are close enough, and so forth. I don't think I have to have an assault weapon to defend my home. I don't see the logic in thinking JQ Public has to have assault weapons. If someone is trying to include shotguns and handguns (and by these I do not mean the small hand-held machine gun types)and some small-caliber rifles that are used for hunting, under the title of "assault weapons," then maybe the fight needs to be on changing the terminology of what is classified as "assault weapons."
I think we citizens can fight for the protection of the First Amendment without including the need for allowing "assault weapons" to be sold to Mr. & Mrs. JQ Public.
But, keep up the good fight. We need people willing to keep people aware.