Hey, SodaPop! It is Football Day, ya know!
The early Colt full autos (weren't they originally called AR-15s before becoming known as M-16s?) were designed for cartridges loaded with IMR powder. The design rate of fire on full auto was roughly 900 rounds/minute.
The Olin Corporation lobbied the Pentagon, to be the supplier of ammo. Their ammo used Ball powder. Its burning characteristics produced a rate of fire of some 1,100 rounds per minute.
Regardless of exact "why" arguments, they did jam in combat situations. As in any argument, fingers were pointed in all directions, including upward with the rigid digit. It was truly a Chinese Cluster Mess.
I have to assume the gas port was reduced in size, in order to reduce the rate of fire. (Today's Ball powder is probably not the same as that of 1965.) Also, the forward assist bolt was developed. As I said before, it's 35 years later, and a lot of development work has been done.
There is a lengthy, detailed article (two parts?) in Soldier Of Fortune magazine, some five-ish or so years back. Names were named, details were given...
While it is not uncommon for people to die in "field testing", I think it's accurate to say that this rifle was put into the field before truly rigorous testing was completed--thus the problems in Vietnam which led to unnecessary deaths. The politics of lobbying didn't help matters.
Hope this helps,
Art