CONDITIONS FOR ALL 3 TESTS: 98 Fahrenheit, 49% rel humidity, 3-7 mph wind from 12-1:30 o'clock directions, level ground, partly cloudy to clear.
TEST GUN: Colt Match Target HBAR .223 Flattop rifle (post-ban AR-15) w/ chrome-lined barrel. Installed rubber AccuWedge at the rear receivers' attaching lug. The gun was not cleaned during the sight-in and shooting tests.
AMMO USED: 500-yd test#1: Hornady V-Max .223 55-gr. Ballistic-tip Moly Coated #83273 (3,240 FPS); 400-yd test#2: Hornady Match .223 75-gr. BTHP Moly Coated #80263 (2,790 FPS); 400-yd test#3: Hornady TAP Precision .223 75-gr. BTHP #80265 (2,790 FPS) coated w/ Birchwood-Casey Moly Lube Aerosol.
SHOOTING REST: Protektor mechanical rest (not rigidly mounted) with front and rear shooting bags.
This 500-yard shooting is an excellent example of what mirage effect can do to accuracy. Before shooting the 500-yard target, I zeroed the scope by shooting 100, 200, 300, & 400-yard targets (one to two shots at each yardage). Although I wasn't trying to zero the elevation at each yardage, the scope shot perfectly zero for windage at each target after zeroing the windage adjustment for the 100-yard target. After shooting at the 400-yard target w/ the reticle perfectly zeroed for windage, I shot the 500-yard target with surprising results.
Looking at the 500-yard through the Vari-X III 3.5-10x40
#45257 scope, I saw that the massive ground heat waves were significantly
distorting the image of the 500-yard target, but not to the point where I
couldn't easily place the crosshair over the bulls-eye. I took 5 shots,
waiting 1 minute between shots. Because my previous 100-400 yard shots
were perfectly zeroed for windage, I was surprised to find that three of my
500-yard shots landed in a tight group (1-15/16") at the right side of the
target. 
I believe that mirage effect brought the group to the right side of the
target. I'm not sure where the other two shots landed, but they probably
went above the 3-shot group.
Besides vertically leveling the rifle before each shot, I also exceeded the scope's eye relief to shrink the image and then center the image on the glass. Shrinking the image and then centering it on the glass seems to reduce parallax and allow for more accuracy, at least it does for me while using Leupold scopes.
After shooting the 500-yard target with surprising results (windage
accuracy),
which I believe was caused by mirage effect, I decided to back off and shoot a
400-yard 5-shot group using Hornady Match .223 75-gr. BTHP Moly Coated
ammo. I didn't make any reticle adjustments after shooting the 500-yd
target, so I was anxious to see if the 400-yard 5-shot group would be
well-centered for windage. I shot this 400-yard 5-shot group waiting
1 minute between shots: 
2-shots= 1/2", 3-shots=1-3/8", 4-shots=3-1/4",
5-shots=4". I also exceeded the scope's eye relief to shrink the
image and then center the shrunken image on the glass, which I believe helps
reduce parallax.
With this group shooting very accurate for windage, it's clear to me that the
previous 500-yard group (test#1) was significantly affected by mirage effect.
After shooting the 400-yard target with good results, I decided to
shoot another
400-yard 5-shot group using Hornady TAP Precision .223 75-gr. BTHP ammo coated
with Birchwood-Casey Moly Coat aerosol. I don't remember if I made any
elevation adjustment beforehand, but I know I didn't make any windage adjustment
for all three shooting tests. I shot this 400-yard 5-shot group with
only a max. 10-second delay between shots, and you can see that there was much
barrel walk caused by the extra barrel heat:
.
Law that forbid ownership of arms disarm only those who are neither inclined, nor determined to commit crimes. Such laws make things worse for the assaulted and better for the assailants. They serve rather to encourage than to prevent homicides, for an unarmed man may be attacked with greater confidence than an armed man. --Thomas Jefferson