Not at all, though they didn't exactly *tumble*. When the bullet nose [ogive] hit a dense material such as flesh or bone, the still-spinning bullet's heavier rear portion continued its turning, often in ever-widening diameters, leaving a wound whose path resembled a funnel. That was a characteristic of the very early 1:14 twist barrels, changed to a 1:12 twist in the M16A1, particularly when the 55-grain M193 ball ammunition was used, which also had a nasty habit of breaking in half at its cannulure [bullet jacket's grooving]once it got sideways while meeting resistance. The diverging wound channels could get real nasty, and could help promote blood loss and shock, giving the old M16 and M16A2 a better reputation for stopping power, if not for mechanical reliability or long-distance accuracy, long ranges not usually being a problem in Vietnam where the usual enemy equipment were AK and SKS rifles or leftover US WWII carbines. But every now and again, someone on the other side would turn up who could really shoot.
Too, some hits were from tracers that had burned out at maximum distance, or had been fired from short-barrelled XM177E2 *Car 15s* that sometimes had the same stopping problems that 14.5-inch barrelled M4s now exhibit. If you're gonna use the 5,56mm, use it in as long a barrel as possible, and take advantage of that accuracy...if it's not a windy day.
-archy-/-