The ignorance/deception in the OP is so palpable that even this life-long civilian can spot some of it.
First, the 5.56 NATO was developed in the mid-late 1950s, well before the Vietnam War.
Second, many/most civilian rounds are versions of military rounds, so the 5.56//.223 round used in the AR-15 type rifles is not unique in this respect.
Third, that the muzzle velocity of the 5.56 NATO is supersonic is not unusual. The 7.62 NATO is similar in muzzle velocity, as is the .308 Winchester civilian round from which the 7.62 NATO was derived. I haven't done the math, but it is very possible that the 7.62 NATO hit the target with greater energy that does a 5.56 NATO.
Fourth, the "tumbling" thingy has been disproved. The large entry wounds that gave rise to the tumbling myth were found to be due to bullet fragmentation.
Fifth, the military's basic issue battle rifle went from the semi-automatic M1 Garand to the full automatic M14 (early 1960s), M16, and M4. Other than for experimenting with the 5.56 NATO round, nothing like an AR-15 was in general use in the military. There are a couple of ways in which, "It is a weapon amazingly similar to the rifle I carried in the US Army," could be a lie. He may be conflating for DU-ignorami the M1 Garand, which does not resemble an AR-15 and fires a different round, and the AR-15. Or he may be conflating the semi-automatic AR-15 with the full automatic (if so selected) M16. Either way, "Mortos" is lying and playing with the general ignorance and confirmation bias of most DU-folk.