IF YOU BELIEVE THAT THE RIGHT TO OWN A GUN IS RESERVED FOR ONLY THE MILITIA, YOU SHOULD READ THE FOLLOWING:

TITLE 10 > Subtitle A > PART I > CHAPTER 13 > Sec. 311.
Sec. 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

TITLE 10 > Subtitle A > PART I > CHAPTER 13 > Sec. 312.
Sec. 312. - Militia duty: exemptions

(a)
The following persons are exempt from militia duty:

(1) The Vice President.

(2)
The judicial and executive officers of the United States, the several States and Territories, and Puerto Rico.

(3)
Members of the armed forces, except members who are not on active duty.

(4)
Customhouse clerks.

(5)
Persons employed by the United States in the transmission of mail.

(6)
Workmen employed in armories, arsenals, and naval shipyards of the United States.

(7)
Pilots on navigable waters.

(8)
Mariners in the sea service of a citizen of, or a merchant in, the United States.

(b)
A person who claims exemption because of religious belief is exempt from militia duty in a combatant capacity, if the conscientious holding of that belief is established under such regulations as the President may prescribe. However, such a person is not exempt from militia duty that the President determines to be noncombatant

Also, note that modifications have been made regarding the upper age limit and inclusion of women.