2124 to LoD
Unless it's changed very recently, 5.56 is(was) the standard NATO rifle and squad automatic weapon round. NATO 7.62 is the standard medium machine gun round.
7.62 x 39 is the old Warsaw pact standard everything round, until you get to heavy and AA machine guns (13mm and 15mm) The current standard Russian round is 5.45 mm, with the introduction of the AK-74M in the 80s.
I haven't kept up with any recent changes, since my soldiering days are done.
Things have quieted down now, but the new guys have a cool weapon - it's a modern remake of the WWII M-2 automatic carbine. Composite materials, collapsible wire stock, .308 caliber (lot more punch than 5.56 mm, without the range or penetration of the 5.56 or 7.62 NATO or 7.62 by 39 rounds, since it's a carbine barrel. Nice street sweeper
, without the excess range and penetration to accidentally kill somebody in their house two blocks away. Looks like a real sweet gun, and the collapsing stock looks a bit like Steyr's or FN's design style.
. BTW, what the Hell is a "collapsible wire stock" 
