Yes, historical context is important, but forget it for a moment. Taken at face value, what does the text mean?
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.
Is the leading clause an essential modifier or an absolute phrase? This is not a political question! Justify your answer with appeals to rules of English usage.
What say you, grammarians?
The only interpretation that would make sense at all would be taking the first two parts as an absolute construction, and the the other two as the main clause. With modern punctuation, it would look like this:
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.
The part in italics is an absolute construction, as it is still used in modern English, to be paraphrased as because a well regulated militia is necessary to the security of a free state...; the main clause speaks for itself.
It seems some modern-day politicians have committed the error of interpreting this sentence in an unhistorical manner: the use of commas has changed quite a bit since the time this sentence was written, and it should be read according to the punctuation conventions of that time.
The use of commas wasn't as well established in 1791 as it is now: the connection between pause and comma was much stronger (commas now mark about 50 % pause and 50 % syntax, depending on the comma). In the 18th century, commas could be used where one heard a pause, more or less, regardless of syntax. In modern English, the comma before being forbids taking what comes before as a single absolute construction together with what comes after; but that does not apply to the 18th century. We shouldn't pay those commas much heed except as marks of pause. It is better to read the sentence out loud and see how we can make sense of it. Any other reading than the above seems vastly inferior.
That said, I find it mystifying why anyone should want to treat any text as an eternal truth, ignoring the historical circumstances that led to its creation. But I suppose this mystery isn't relevant here.
[Edited:] The fact that punctuation was quite different may be observed in the fifth article of the same Bill of Rights; according to most modern grammarians and style books, at least the first two commas would be superfluous and wrong:
No soldier shall, in time of peace be quartered in any house, without the consent of the owner, nor in time of war, but in a manner to be prescribed by law.
The following is from Punctuation in English since 1600 (archived version), a little essay by John Wilkins of Ohio State University. It has presumably been incorporated into the Encyclopaedia Britannica. The author seems knowledgeable and reliable.
Excessive punctuation was common in the 18th century: at its worst it used commas with every subordinate clause and separable phrase. Vestiges of this attitude are found in a handbook published in London as late as 1880.
Furthermore, Wikipedia mentions that, at the time, the States passed a version of the Bill that had the exact modern punctuation: the version in question is the one that was passed by Congress.
This shows how different and varying the use of commas was in the 18th century. Only after 1900 did most writers' punctuation come close to ours, though the comma's move from pure pause to mainly syntactical mark can be traced back to the 17th century, or even earlier.
[Edited again:] By contrast, some anti-gun politicians have tried to read the sentence as follows:
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.
They take the two phrases in brackets as modifying a well regulated Militia: "a well regulated militia, which is necessary to the security of a free state—in other words, the right of the people to keep and bear arms—shall not be infringed."
Following that interpretation, it is this well regulated militia of a State that is not to be infringed upon by the federal government. The italicized phrase about the arms is then to be taken as a non-essential, further explanation of what this militia is about. This reading is inferior for several reasons:
The incorrect reading can be found in the Yessky Brief (September 1999) for U.S. v. Emerson, 12 note 4:
The grammatical effect of these two unusual commas is to link "A well regulated Militia" to "shall not be infringed" to emphasize, in other words, that the goal of the Amendment is to protect the militia against federal interference.
Language Log cites the above and agrees that it is nonsense.
Cerberus is certainly correct in interpreting the language of this sentence as though the first and third commas did not exist. For the sake of comparison, here is the opening paragraph of the U.S. Congress's 1789 proposal of twelve articles of the Bill of Rights (the third through twelfth articles eventually became the first through tenth amendments) for ratification by the states:
The Conventions of a number of States, having at the time of their adopting the Constitution, expressed a desire, in order to prevent misconstruction or abuse of its powers, that further declaratory and restrictive clauses should be added: And as extending the ground of public confidence in the Government, will best ensure the beneficent ends of its institution.
The punctuation used, is clearly, comma crazy, to say nothing, of the colon.
The interesting thing to me about the wording of the second amendment is the complication that the framers introduced in their effort to justify the stated right. No other amendment in the Bill of Rights includes a rationale for why it should be made part of the law of the land.
It's not that the right to keep and bear arms is stated more equivocally than the other rights in the Bill of Rights—"the right of the people to keep and bear arms, shall not be infringed" is about as unequivocal as you can get. And yet the introductory explanation does affect our sense of the right to some extent. The reason underlying the right isn't that "A potent revolutionary force capable of overthrowing an unjust government, being necessary to the security of the people of a state, who desire to remain free, if the government should become tyrannical, ..." If it were, then presumably the right would have been expanded to cover "weaponry and ordnance of all kinds."
The rationale (in 1789) for the right is that the nation needs a well-regulated militia; but the right itself is very nearly absolute (at least in the abstract; under actual U.S. law, felons in some jurisdictions forfeit that right—as well as the right to vote—even after they've served their term of incarceration, though they presumably do not forfeit their right to free speech, their right to free exercise of religion, and their right not to have soldiers quartered in their homes during peacetime, for example).
Whether you view the "A well regulated militia" rationale as a literary flourish that unfortunately impedes clear understanding of the right to keep and bear arms or whether you view that rationale as an opportunity to reconsider whether the right itself should be revisited in view of the rationale's arguable obsoleteness will depend (probably) on how you feel about keeping and bearing guns yourself and being surrounded by other people who keep and bear them.
The basic problem is that to the modern reader the sentence is ungrammatical, so you are left to investigate historical context (grammatical and other). It is of the form:
Noun phrase, subordinate clause, noun phrase, subject-less verb phrase.
Subordinate clauses, by their nature, can be dropped for analyses like this. So you have NP, NP, VP, but because of the commas neither NP can stand as the subject of the VP. The sentence (if you could call it that) does not make sense as written under modern rules of grammar.
If the first and last commas were omitted you would have a valid sentence:
Subordinate clause, the right of the people to keep and bear arms shall not be infringed.
That doesn't mean that's what they meant, of course. You asked about modern interpretation.
It is completely ambiguous, which is why this piece of drafting has been so contentious. It's an example of incredibly poor drafting.
The main reason to interpret this as being an individual right is that if it is intended that "A well regulated Militia, being necessary to the security of a free State" limits the following phrase, it is entirely unclear what right would be so declared. What relationship between militias (well-regulated or not) and the keeping and bearing of arms does it declare?
Over all, it looks like the first two clauses are intended to limit the rest of the sentence, but because that doesn't actually make sense, and legal interpretation would rather leave one part of the sentence as surplusage, rather than leave the whole sentence as nonsense, it ends up being capable of being intelligble only if the first two clauses are merely declaratory.
