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. [United States Constitution, Amendment II]
A well regulated Militia
What is a militia? From dictionary.com:
1. a body of citizens enrolled for military service, and called out periodically for drill but serving full time only in emergencies.
2. a body of citizen soldiers as distinguished from professional soldiers.
3. all able-bodied males considered by law eligible for military service. [ref]
This basic understanding voids the argument that the Second Amendment guarantee of a right to keep and bear arms only applies to "modern militia" members of the National Guard or other government-sponsored and supplied military groups.
What about "well Regulated"? The Founders' idea of "well regulated" definitely wasn't thousands or even hundreds of pages of laws and bureaucracy. Their meaning was "properly organized", "orderly". That's sensible. Besides, they had to say something. "A Militia" by itself would have been a lot less eloquent.
being necessary to the security of a free State
This vital declaration separates two very important phrases, and applies equally to both. It is the "why" of the Second Amendment.
Above all else, the Founders were interested in securing freedom for themselves and their posterity. They knew, from first-hand experience, what conditions were necessary to secure and maintain a free state.
You can read the Second Amendment both of these ways:
"A well regulated Militia, being necessary to the security of a free State, shall not be infringed."
"Being necessary to the security of a free State, the right of the people to keep and bear Arms shall not be infringed."
A well Regulated Militia is necessary to the security of a free State. The right of the people to keep and bear Arms is also necessary to the security of a free State. Both are necessary.
(For a more detailed treatment of this topic, see my related post, Why the Second Amendment is Second.)
the right of the people to keep and bear Arms
Right is a powerful, meaningful word, one not used lightly by the Founders.
A right is "a just claim or title, whether legal, prescriptive, or moral". It is "that which is morally, legally, or ethically proper", a "moral, ethical, or legal principle considered as an underlying cause of truth, justice, morality, or ethics." [ref]
Arms is short for "armaments". Armaments is a general term for weapons, which in the 1700s included rifles, pistols, muskets, knives, swords, and yes, even cannons and mortars. The Founders intentionally didn't attempt to enumerate particular armaments. They opted instead for the very broad term "Arms".
To keep means to have in one's private possession. To own, to safeguard.
To bear means to hold, carry, wield or use. The Founders didn't intend for citizens to have to keep their armaments under lock and key.
The Founders believed that it is just and moral for people — ordinary people like you and I — to own, and to carry, and to use Arms. They made sure that the Second Amendment guaranteed that right.
shall not be infringed
The final key to understanding the Second Amendment is the word "infringed".
The English word infringe comes from the Latin infringere, which means to weaken [ref]. To infringe is to encroach, to "advance beyond proper, established, or usual limits; make gradual inroads", or to "trespass upon the property, domain, or rights of another, especially by stealthily or by gradual advances." [ref, emphasis added]
"Shall not be infringed" is crystal clear. The Founders' intent was that the government do nothing to even slightly weaken or violate in any way the right to keep and bear Arms.
Even though the Founders knew that scientific and technological innovations would eventually result in more powerful and lethal armaments, they chose not to impose any limits.
If the Gatling machine gun, invented in the mid 1800s, had been around in the 1700s, would the Second Amendment have excluded it? What about hand grenades? Rocket launchers? Tanks? Atomic weapons? We can speculate, but we don't know.
What we do know is that the Founders prescribed a mechanism for changing the Second Amendment: Constitutional Amendments. Instead, our government has increasingly infringed on the Second Amendment using unconstitutional legislation and regulation.
Background checks. Licensing. Registration. Permits. Mandatory locks. Gun-free zones. I could vote for some of these in some cases — if there was a Constitutional Amendment authorizing infringement on the right to keep and bear Arms.
But there is not. Which begs the question: What part of "shall not be infringed" do they not understand?