A militia is not a standing army, true. It is not composed of full-time professional soldiers, but, as you point out, by civilians subject to call-up at times of national emergency, or local disturbance. In the 18th century, these were understood to be (in order of likelihood), Indian hostility, insurrection or rebellion, and lastly, foreign invasion.

But the key is that the militia is an arm of local government, sanctioned by and answering to the State, and subject to military organization, discipline and command. This is what is meant by "well-regulated militia". And remember, the purpose of a milita is the "security of the free State", not the political fantasies of individuals who have single-handedly appointed themselves to one. To the founders, what we call militias today, either the gangs of armed thugs you see in Baghdad or rural America, are NOT militias. They're the reason our founders felt we needed militias.

Militia troops have military rank, wear uniforms with insignia, swear an oath, follow orders, and are formally mustered unto organized formations which drill and fight under the command of legally commissioned officers appointed by the State. There is a reason the military relies so much on tradition, ritual, custom, and ceremony. It is to impress on its members the legal legitimacy of its mission. Wrapping yourself in the flag and buying an assault rifle and a camouflage suit does not make you a militia member. It certainly doesn't magically transform you into a patriot, either.

I've always felt it was a waste of time to try and imagine what the founders meant to say, or what they were thinking; we are not simultaneously mind readers and time-travelers. Even their writings differ and are open to interpretation (they often disagreed violently amongst themselves!). Freedom, civilization and common sense demand we rely instead on what survives in law and statute, and how it is interpreted and amended by legislature and the courts.

Going out and buying a gun may be a legal act, but it does not by itself give you the right to be an American soldier and to take a lives on behalf, or with the authority, of the nation. Those who insist it does are nothing but common criminals determined to impose their ideology and prejudices by force on anyone else who might tend to disagree with them.