Due to an Executive Order signed by President Clinton, the US military can no longer "surplus" ammunition, except via the Civilian Marksmanship Program (CMP), and the CMP does not sell 5.56/.223 ammo. US military ammo (most notably from the Lake City Army Ammunition Plant) used to be widely available, but has become quite scarce and the prices quite high, in the last few years. Though some military ammo components are "saved" by contractors disassembling the ammo and selling the components, most expired or out-of-spec lots of ammo are burned. Billions of rounds of ammo, paid for by US tax dollars, are burned yearly.
Small quantities of loaded ammo are occasionally available, usually at gun shows. This is either old stock from before the ban, police trade-ins, or stolen military ammo. The military has programs where it gives police departments surplus weapons and ammo and these departments sometimes trade this ammo to their ammo distributor for cash or other ammo. The distributors then make the exchanged military ammo available to the civilian market. Quantities are usually small and prices high.
Despite this, military specification ammo (Mil-Spec) is available to the public and can be obtained from a variety of sources.
Opinions: AR15.COM members have had success with the following dealers, among others: