I was a Light weapons infantryman with B Co. 2/5, 1st Air Cav, 4JUL66-30JUN67 in beautiful SE Asia...
I carried three different M-16s.
One jammed intermittently(1 to 11 rounds, before the case rim was ripped off by the extractor, but the brass case remaining stuck in the chamber),
The second jammed three different ways(spinning the empty case in the breach, blocking the next round; the bolt would lock back; and the the usual one of the fired case hanging in the chamber).
The third one worked flawlessly.
John Fowles wrote a book, "National Defense", one section dealing with the M-16.
Army Ordinance was peeved that it had not been consulted on the development by Armalite of the AR-15, demanded 1) the use of Ball Powder rather than the two part IMR cleaner burning powder(developed by Armalite for the AR-15), and 2) that the muzzle velocity be increased from 2800 to 3100 feet per second.
The first demand caused fouling of the gas ports, and the second exponentially increasing the chamber pressure, thus over-expanding the tiny .223 (5.56mm) brass, causing it to hang in the chamber.
My personal take is that during the rapid build-up at the start of the war(62-67) the various manufacturers, some in their haste, bored the chambers with little quality control.
Their solution was to have a series of bolts, whose head-space(between the back of the brass and the face of the bolt) was cut to compensate for variations in the bores, and assemble them, matching each individually. This was faster than re-boring, or in the case of over-boring, resizing the barrel and re-boring, thus resulting in inconsistent under-boring, shortening the chamber, and was the norm.
Of course, when GIs cleaned their weapons, it was often in a tin-pot(helmet) full of JP4(kerosene) in which the various bolt mechanisms, from various manufactures, were tossed into, then randomly extracted, thus mismatching the bolts and chambers.
This inconstancy in finished product becoming one of the major causes of malfunction.
I have spoken to a number of GIs from Vietnam who used valve grinding compound and/ or dirt and oil and enlarged the chambers using a cleaning rod and fired brass to remedy the chamber hanging problem.
When I arrived, we had no cleaning rods, I sent home for one, and was very popular for a while until our CO saw me, had a fit, and "got on the horn" to the REMFs, and finally got us all the supplementary "kits" with cleaning rods that the those in the rear, who NEVER fired their weapons, were appropriating. These included bipods which were useless and were tossed immediately.
Previously, when the brass would hang in the chamber, we were using coat hangers to try to knock the brass out.
We were, effectively, using muzzle-loaders against the AK-47s.
In addition, there is the popular misconception, including myself for many years, that most of those who went to Vietnam were conscripts(I being one).
Two-thirds, 67% were volunteers. That is: their service numbers, in the Army, were RA (Regular Army) not US (Draftees). The overwhelming majority of Marines were volunteers as well.
I have copies of my Company rosters, an infantry line unit, for four of the twelve months that I was in-country, and they reflect that same 67 percent RA, 33% US.