They started entering the inventory at the end of the 1960's. They were rare in Vietnam but became more common in the 70's.

..'79-'83, we had both 20's and 30's. Most of us, myself included, preferred the 20 rnd mags as they were more reliable.

The Russians designed the AK to be used in a submachine gun role, hence the 30 rounders. Since the fighting in Vietnam was up close and quick, we came up with our 30 rounders.

A buddy of mine was the company armorer, and I spent a lot of time assisting him whenever IG inspections were coming up. Trust me, disassembling and cleaning several footlockers of magazines is not
an enjoyable way to spend the day.

I was attending an NCO training academy that involved a great deal of field exercises and instruction.
The 30 round magazines were just starting to trickle in, and my buddy issued me 6 brand spanking new mags and 30 round mag pouches. The cadre, course instructors were impressed as even some of them had never seen or used 30 round mags themselves.

in the 1990s when I was in Korea. I asked the armorer if they had any 20s left because my arms are too short to fire from prone position, and it seemed harder to secure in the rifle. I agree with your assessment.

The first 30 rounders came about in 1969 and only available on a trial basis to SOG, LRRP, and SEALs as they had the ability to procure prototype or experimental weapons.

The 30 rounders were a lot more reliable than the 20 round mags that we could only load 18 rounds in. I think it was the curved design and a better follower, but I'm not sure why, they just worked better.

Never had a jam with one, as long as you kept the feed lips from getting all bent.

rock solid feeding from a poly mag.

My 20 pmags are my favorite for hunting and target shooting. But the 30 are fun for plinking, and I like the 10 extra lifesaving rounds the 30's provide for home defense.

We recieved mags and pouches seven days before deploying.

only holds 8. The 'standard' went up from there. Would you have rather have had only 8 rnd mags in your AR?
I know its kind of a BS comparison, clip and magazine. 30-06 and 5.56. But then again 30 5.56 rnds is almost as good as 8 '06 rounds. Still think the M14 was the best service rifle. Period

I was only thinking about reliability. I have never shot a thirty round mag. I did tape two twenties back to back.
The M16/AR15 is quite the weapon for shooting fast and accurately.