From the nation's inception, Citizens have been responsible for defense as 
repeatedly defined in the Laws of the United States of America.

Under current US law, we the people have an obligation to be armed with weaponry customary to our timeframe and trained in the use of those arms. The Militia Act and the current US Code definition of the militia includes all able-bodied adult males. As originally passed, these laws require more than simple inclusion in the militia. These laws specify that, at our own expense, we are to have and maintain customary weaponry of our timeframe, in good and operable condition, with ammunition and attire (accoutrement) necessary to assemble a fighting defense force.

Furthermore, the US Supreme Court, in US v. Miller (1939), incorporated the concept of militia-type weapons as being weaponry commonly in use for defense and whose ownership is protected by the Second Amendment.

The Second Amendment to the Constitution of the United States
Ratified 17 December 1791
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 Statutes at Large/Volume 1/2nd Congress/1st Session/Chapter 33
(aka The Militia Act of 1792, enacted 8 May 1792 & basis for 10 USC §311)
Section 1. Be it enacted by the Senate and House of Representatives of the United States of America in Congress assembled, That each and every free able-bodied white male citizen of the respective states, resident therein, who is or shall be of the age of eighteen years, and under the age of forty-five years (except as is herein after excepted) shall severally and respectively be enrolled in the militia by the captain or commanding officer of the company, within whose bounds such citizen shall reside, and that within twelve months after the passing of this act. And it shall at all times hereafter be the duty of every such captain or commanding officer of a company to enrol every such citizen, as aforesaid, and also those who shall, from time to time, arrive at the age of eighteen years, or being of the age of eighteen years and under the age of forty-five years (except as before excepted) shall come to reside within his bounds; and shall without delay notify such citizen of the said enrolment, by a proper non-commissioned officer of the company, by whom such notice may be proved. That every citizen so enrolled and notified, shall, within six months thereafter, provide himself with a good musket or firelock, a sufficient bayonet and belt, two spare flints, and a knapsack, a pouch with a box therein to contain not less than twenty-four cartridges, suited to the bore of his musket or firelock, each cartridge to contain a proper quantity of powder and ball: or with a good rifle, knapsack, shot-pouch and powder-horn, twenty balls suited to the bore of his rifle, and a quarter of a pound of powder; and shall appear, so armed, accoutred and provided, when called out to exercise, or into service, except, that when called out on company days to exercise only, he may appear without a knapsack. That the commissioned officers shall severally be armed with a sword or hanger and espontoon, and that from and after five years from the passing of this act, all muskets for arming the militia as herein required, shall be of bores sufficient for balls of the eighteenth part of a pound. And every citizen so enrolled, and providing himself with the arms, ammunition and accoutrements required as aforesaid, shall hold the same exempted from all suits, distresses, executions or sales, for debt or for the payment of taxes.

[[ EXCERPTS FROM SECTION 4 ]] Sec. 4. And be it further enacted, ... shall furnish himself with all the equipments of a private in the infantry, until proper ordnance and field artillery is provided... The commissioned officers to furnish themselves with... Each dragoon to furnish himself with... [with uniforms] to be furnished at their own expense; the colour and fashion to be determined by the brigadier commanding the brigade to which they belong.

10 USC §311 as of 2 December 2015
§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.

U.S. v. Miller (1939)
307 US 74 (1939)
Frank Layton and Jack Miller were charged with violating the 1934 National Firearms Act, which regulated and taxed the transfer of certain types of firearms, and required the registration of such arms. The Miller court decided the following:

1. The National Firearms Act was not an unconstitutional usurpation of police power reserved to the states.
2. "In the absence of evidence tending to show that possession or use of a 'shotgun having a barrel of less than 18 inches in length,' which is the subject of regulation and taxation by the National Firearms Act of June 26, 1934, has some reasonable relationship to the preservation or efficiency of a well-regulated militia, it cannot be said the the Second Amendment to the Federal Constitution guarantees the right to keep and bear such an instrument, or that the statute violates such constitutional provision."
3. "It is not within judicial notice that a shotgun having a barrel of less than 18 inches in length is any part of the ordinary military equipment or that its use could contribute to the common defense."
4. "The Second Amendment must be interpreted and applied with a view to its purpose of rendering effective the Militia."

Note that in the entire text of Miller, neither the words "state militia" nor "National Guard" are to be found.

Regarding item 4) above, the Miller court defined the Militia as the following:

The signification attributed to the term Militia appears from the debates in the Convention, the history and legislation of Colonies and States, and the writings of approved commentators. These show plainly enough that the Militia comprised all males physically capable of acting in concert for the common defense.

The Supreme Court reversed and remanded the case back to the district court, giving the defendants a chance to provide evidence that a short-barreled shotgun could contribute to "the efficiency of a well-regulated militia." Note Miller only required evidence that the weapon contribute to the efficiency of a well-regulated militia. The court never said the defendants had to belong to a well-regulated militia. In other words the Miller case interpreted the Second Amendment to mean one has the right to own militia type weapons.
(Click here for the SCOTUS case)