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.
Is the First Amendment obsolete because they didn't envision the computer, the telephone, TV, or the high speed "assault' printing press?
The flintlock of 1776 was the AR-15 of today. The SCOTUS 1939 Miller decision that is highly touted by liberals, established the MILITIA purpose of weapons supported by the Second Amendment. The sawed off shotgun in the case was not then recognized for it's value in warfare.
We are to have military type arms if we are to withstand the tyranny of an out of control government. That was the purpose of the Amendment (and that of 35 or 40 of the state constitutions) in the Federal Constitution.