Papa,  I know of no requirement for militia membership before buying a gun,  but there is a very high correlation between gun buyers and membership in the unorganized militia.  10 USC 311 defines the unorganized militia of the US as being all able-bodied males ages 17 to 45.  The various state laws typically define the age group for their unorganized militias as 17-65.  So the vast majority of guys buying guns are part of the unorganized militia (even if they don't know it).