despite 2 pages of "yes" answers, I'm going to go with "not really". And I say that even though I have a Distinguished badge shooting the AR 20" service rifle in the National Match course with iron sights out to 600.
You've asked several questions, and they should really be answered individually:
Is it possible to reach out to 500 yards? yes, of course. the bullet will fly for miles.
Is it EFFECTIVE? that's a different story. If you're trying to kill someone or a game animal, keep in mind the M193 and M855 ammo is designed to fragment, not expand. To reliably fragment, it needs to be going a certain velocity, and M193 out of a 20" barrel drops below that at about 200 yards. data and pictures here:
https://www.ar15.com/forums/AR-15/M...rom-the-past-Posted-for-Posterity-/16-731116/
otoh, if you're just harassing your enemy, or wounding them by poking a 22 cal hole, you can "effectively" do that much further away
How realistically possible is it for a solo shooter in a field type situation, with an iron-sighted 20" AR to hit a man sized target at 500 yards?
Not bloodly likely
I used to shoot about 197-198 10x at 600, which is 17-18 out of 20 rounds in a 12" circle, and half of them in a 6" circle at 600. but....
First, shooting a 36 inch black bullseye on a 6 foot square white backer is easy, because you can SEE it. so you can put that circle on top of the front sight post or hold line of white or flat tire or whatever. But outside walmart, most people are substantially smaller than 36" wide and they're not wearing black on a white background. they'll be wearing camo, or earth tone outdoor wear. and they'll prob be hiding behind cover. Maybe you can see them because they're standing in a window or doorway. But you can't use the pumpkin on a post method. You'll have to use a zero that tries to put the bullet right on top of the front sight, which means your sight picture will be covering half the target, so what you're looking at gets a lot smaller. i.e. if you're aiming at the middle of your man sized target, you'll only see half of it. which means it's harder to see.
Next, even though you stipulate 500 yards, the shooter in a field situation won't know that unless they have a laser range finder. People, even highly trained military guys, generally suck at range estimation. And honestly, I'd be surprised if 1 in 100 AR owners have a LRF and practice with it. Heck, I'd bet more than half of PRS shooters don't have one because ranges are given to them at matches.
And, field positions suck. Tall grass, shooting off stumps or fence posts or whatever. You're not going to have your shooting jacket. Prob won't have time to sling up. Giving the "skilled" shooter the benefit of the doubt, they can get into natural point of aim, as long as they don't have to track a moving target. but you prob won't be laying prone or shooting from a benchrest, so your ability to hold a man sized wobble zone will be greatly diminished.
What goes in to judging windage at such a long range and is it even realistically possible without range flags and spotters?
Depends on terrain and seasons. Shooting in the mountains from one ridge to another in the winter when there aren't any leaves? pretty dang hard. If the terrain is flat and you have good indicators, like grass blowing etc, then it's pretty easy, although iron sights eliminates one of the best indicators which is mirage.
But yes, most of the time and in most circumstances, it's pretty easy to judge the wind.
HOWEVER, if you miss, and you don't have a spotter, and you're using iron sights, you won't be able to see where you missed or even if you hit/missed. So it's dang hard to make corrections and getting a 2nd round hit like you could with a scoped rifle.
