We have one. Article 1, Section 8 of the Constitution empowers Congress, and it says:
"The Congress shall have Power 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". By this power, Congress and only Congress can decide what "well-regulated" means with regards to the miltia. They've done so five times in our history in the various Militia Acts. The latest was in 1903, and is embodied in US law by 10 USC 311:
§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.
There is not nor has there ever been a requirement to be in the militia to own a gun, and interestingly you don't have to own a gun to be in the militia.