In the early days, a decentralized force of farmers, shop owners, blacksmiths, leather workers, and common folk took on the most powerful military in the world. They bought the infant republic time to assemble and train a professional army and eventually win their independence. At the time, battles were fought in long skirmish lines firing volley after volley back and forth. People being people, the battles wore on for hours, and surprisingly little killing actually occurred for the number of rounds expended. To get the full context of what I'm talking about, read "On Killing" by LtCol Grossman. These battle took place before the warrior class was effectively conditioned to kill on command. There is ample evidence that upwards of 95% of soldiers would either refuse to fire on their fellow man, or simply miss on purpose.
Today- that's not the case. Our professional military is much more capable of killing than at any time previous. If you put one squad of Marines (that's 13 modern equipped Marines with nine rifles, three grenade launchers, and three automatic rifles) against a company of Red Coats from 1775- the Red Coats would be obliterated. Not just because of the advances in weaponry, but because of the advances in training and conditioning. Of those 13 Marines, one or two wouldn't try to hit, but they'd fire they're weapons for sure, and they'd likely kill to protect themselves and their brothers. The rest WOULD try to kill the Red Coats. That's eleven Marines, with a greater max effective range and the tools and conditioning necessary to put lead on target firing faster and with greater accuracy against well drilled but less conditioned men firing slower firing, shorter ranged, less accurate weapons.
No contest. Marines win. Period.
But what does that matter now?
"A well regulated Militia, being necessary to the security of a free State, the right of the people to keep and bear Arms, shall not be infringed." We read it, hear it, and seldom think in context to what it really states. At the time, the muskets carried by regular citizens were the equal (and in some respects superior) to the standard arms of many nations. The Brown Bess was smooth bored, and many colonists possessed rifled muskets. This gave them the edge in range and accuracy. And whereas the British had the advantage of training and drill, they lacked the capacity for decentralized ops that the Minutemen fell into out of necessity. The nature of warfare was very different then than it is today, and the tools and training have advanced with the theory and practice of war.
This is where the idea of the AR-15 and it's relevance with regards to the second amendment comes into play. The AR-15 isn't a military arm. It's not even a copy of a military arm. The M-16/M-4 is the military version of the AR-15. The M-14, M-1 Garand, M-1 Carbine, 1911, Mauser rifles, 1903 Springfield- THOSE are military pattern weapons with civilian equivalents. The AR-15 is a civilian weapon. It's the modern day equivalent of the Minuteman's musket. Equal to or better than the standard arm of the military, it bridges the gap and grants the average citizen the basic means to serve his liberties when the situation requires.
But the military has some interesting toys of their own. Fully automatic grenade launchers. Heavy, medium, and light machine guns. Artillery. Aircraft of all types. Guided missiles and bombs. Unguided rockets. Nukes. And above all limitless money to purchase said items. The people have whatever they can put toward their arms- and heavy restrictions on what they my own and carry. Pistols and rifles is the norm for the average citizen. While those arms equal those of the regular DoD warrior, they are insufficient to the task of taking on the myriad weapons that the government can bring to bear.
Some would argue that it's insane to think that a force of people armed only with semi automatic rifles can hope to dent a force commanding smart bombs, helicopters, and artillery operated by trained and competent soldiers. These people will then watch movies like "The Hunger Games," "Divergent," or "Magnificent Seven" and blissfully ignore the truth that is staring them in the face. That FREE people possess the tools to mold their own destiny, and that subjects do not. The founders knew this, and codified this truth into law.
To follow this train of thought to it's logical conclusion, I would argue that the second amendment, if we are to interpret it to protect any single class of arms at all, would protect military grade weapons intended for warfare and not those weapons designed for hunting or sport. This idea, of course, would mean that the "sporting purposes" provision that the BATF uses would be backwards, and that "assault weapons" (a bullshit term anyway) would be explicitly protected. In fact, if we move that direction, the automatic weapons ban enacted in 1986 would be unconstitutional and must be reversed. And I'd be ok with that.