My favorite AR is a 6720 upper, by a long distance. That feels most balanced and carbine like in my hands. The 6520 likely has much of that utilitarian feel and inherent shootability.
I had some limited time with A1 uppers in the Guard. By my time there seemed to be an occasional mixmaster A2 uppers and A1 lower in support units. I did not know enough back then to take pictures or detail what I was seeing, but the giggle switch, silvery wear and the various grey shades made them easy to recall. The thin barrel balance just made carrying so much easier and aside from a bit more stability from a heavier barrel, the A2 barrel never changed qualification scores for me.
To date myself, I recall running into an MP unit at Fort Lewis that had just got issued the M4 carbine (all A2 features) because they were some rapid deployment force. Having an M4 was very hot and elite then, so asked to play around with them. I recall being not too impressed, as the stock wobble was terrible and the while shorter than the A2 musket, the heavy barrel balance was still terrible.
My initial training experiences in with the M16 included XM-16s and M16E1s that had been updated to the M16A1 standard (or as close as they could get to it, since you can't do some things like adding the mag release fence to an older receiver).
I suspect some of those rifles would have had some interesting stories of they could talk.
I was USMC reserve and in the PLC track, but without an active reserve unit near my college, so the USMC had me take Army ROTC. Since I could also fit it in, they had me go to the Army Officer Basic course as well. It was...well...interesting. There were big differences in how they train officers to lead and think. The marksmanship training was also significantly different.
In addition to the normal PLC related training, I'd also shot small bore and service rifle competition a couple years before attending the US Army Basic course, on top of growing up on a ranch and shooting prairie dogs since I was 6. Consequently, I knew how to shoot.
However, the M16A1 (actually built on an XM-16 lower receiver) the US Army had issued me would not reliably extract, with the result that I could not zero it. I was recycled into another relay and was issued a different M16A1 (this one built on an M16E1 lower receiver).
Unfortunately, it had apparently been used to open a crate or some similar use (the only thing in my opinion the M16A2 did better than the A1) in the past as the barrel was bent. Even with full left windage applied it was still shooting a couple inches right at 25 meters. I debated just using some Kentucky windage to "zero", but I did not want to stuck with that *** for the rest of the course. As I more or less anticipated the sergeant in charge ignored the fact that while the group was a couple inches right of target, it was a one hole group and decided as a two time loser on his range, I must be the problem.
As a two time loser I was cycled through their "weaponeer" training device. It was an mock up of a M16A1 firing a laser with a stick connected to the muzzle to simulate recoil. On the operators screen you could see where the laser was pointing while the shooter aimed the shot, and then a permanent dot would appear on the target when the shooter "fired". I spent about 60 minutes in line watching shooter after shooter take their turn, with the laser jumping all over the operators screen with commensurately horrible groups. Sometimes getting marginally less bad as they tried to coach them to a point they could potentially keep 5 out of 6 shots inside a 4 cm circle at 25 meters (a very low 5.5 MOA bar).
When it was finally my turn, I assumed the specified fox hole prone postion and started firing my 3 shot groups. After about the 4th shot the operator started applying percussive maintenance to his display as the dot wasn't moving at all on the screen. After 6 shots, the operator was starting to tell me I'd have to shoot again after he fixed his display, when someone behind us started chuckling. We turned to find the battalion commander watching. He told the operator not worry about it and asked me where I'd learned to shoot and how I'd ended up at the weaponeer. I told him I had previous experience in bore and service rifle competition experience, but that I'd been issued 2 rifles that were not serviceable, had been unable to zero, and been sent to the weaponeer to "learn" how to shoot. He laughed and told the armorer I could obviously shoot and to issue me "one of the good ones".
That was an actual M16A1 in decent shape, and I easily shot expert with it. I really enjoyed shooting it on the Army's train fire ranges.
