Isn't your name the dimension of an M-16A2 round?
An M-16A2 round is 5.56 mm, NATO standard round.
Mine is the M14 NATO round, AR-10A4 in .308cal , with Leupold Glass, of which I have several Class III units: full-auto, 3-round burst, short barrel, silencer etc. I carried one of these in Nam for my 3 tours.
More info, here.