Second Amendment Explained:

Amendment II
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.

But, what does it mean?  It means that the people may be armed, and really has nothing to do with regulating the militia.

This is the part of the Constitution that allows Congress to regulate the militia and what the 2d amendment is referring to with "well regulated militia":

To declare war, grant letters of marque and reprisal, and make rules concerning captures on land and water;

To raise and support armies, but no appropriation of money to that use shall be for a longer term than two years;

To provide and maintain a navy;

To make rules for the government and regulation of the land and naval forces;

To provide for calling forth the militia to execute the laws of the union, suppress insurrections and repel invasions;


To provide for organizing, arming, and disciplining, the militia, and for governing such part of them as may be employed in the service of the United States, reserving to the states respectively, the appointment of the officers, and the authority of training the militia according to the discipline prescribed by Congress;

Titles 10, 32, and 50 are the United States Code (federal law) that Congress created pursuant to the Constitutional responsibility "To provide for organizing, arming, and disciplining, the militia."  And from Title 10, Congress provides this explanation for who the militia actually is:

§ 311. Militia: composition and classes

(a) The militia of the United States consists of all able-bodied males at least 17 years of age and, except as provided in section 313 of title 32, under 45 years of age who are, or who have made a declaration of intention to become, citizens of the United States and of female citizens of the United States who are members of the National Guard.
(b) The classes of the militia are—
(1) the organized militia, which consists of the National Guard and the Naval Militia; and
(2) the unorganized militia, which consists of the members of the militia who are not members of the National Guard or the Naval Militia.

It is hard to imagine that "well regulated" means something beyond organizing, arming, and disciplining.  The Second Amendment merely clarifies that since the already Constitutionally provided for well regulated militia is necessary, the people have the right to keep and bear arms so that the militia will have armed people to call upon.  It also clearly separates out the right to be armed from simply militia members to include all of the people and not just able bodied males, just as the First Amendment reference to people assembling means the people gathering, not the militia going to drill.