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“They were like ‘no you’ll be OK, you were just shot in the arm’ but I could feel it in my chest,” Makes sense. If the round impacts on or near the brachial artery, (in the arm) the resulting blood overpressure pulse can stun or damage heart valves.
When the M-16 was first introduced into Vietnam, its (then) "slow" rifling twist allowed the bullets to tumble if perturbed in flight. There were several instances of Viet Cong being killed by nothing but an arm hit.
The VCs' fear of "the Black Rifle" soon spread, and attained near-epic proportions.
Then, to improve accuracy, the Army issued all following M-16's with a "faster" rifling twist -- and its lethality dropped considerably...
TXnMA
Not exactly. The change in the rifling was also due to the fact that dense air (such as cold air in the lower parts of the Vietnamese mountain ranges) would sometimes cause some rounds to deflect and tumble. So would brush, wicker hats and most infamously grass. *Grass* would cause the rounds to deflect and go god-knows-where. Great if you’re doing area suppression, not good when you need to deliver rounds on target. Post-war research shows that after they got over the initial shock, the VC and NVA found that the black rifle was actually killing *fewer* of them, just that the ones who were hit were often taken out in more dramatic fashion due to the round’s instability. It’s worth noting that the VC, who always wanted weapons, despite the availability of captured M16s and ammo often just left them in the field after the initial panic subsided. US and allied troops exceedingly rarely found themselves facing captured M16s.
More reading here: http://www.gunrightsmedia.com/showthread.php?413583-Tumbling-223-controversy-another-view