Rifle Sights not Plumb ???

Often we hear shooters talking about their windage zeros being different at different distances.  It's not uncommon to have a 600 yd no-wind zero that is two min. of angle different than our no-wind zero for 200 yds.  There are two things most responsible for these differences due to change in distance. 
First is the fact that we mostly shoot Sitting Rapid fire at 200 yds and we all tend to cant (tilt) the gun a little clockwise if we are right handed (counter clockwise if left handed) due to a natural tendancy to lean over a little in the sitting pos.  At the 300 and 600 yd line, we are shooting from the prone position, where a right handed person will tend to cant (tilt) a little into his position (counter clockwise) and therefore will have more right windage (or less left windage) on the gun than in the Sitting position. 
Second is the possibility that the front sight is not on the gun straight in line with the rear sight and therefore the rear sight when raised and lowered does not follow the plumb line of the front sight.  We see this on the target as the strike of the bullets straying farther and farther right or left  as we run the sights up from 200 yd  zeros to 600 or 1000 yd zeros.  We cant rotate the rear sight to be at Top Dead Center, on a service rifle, so when shooting prone we must just realize that we may need to add windage as the sights are raised to compensate for the off verticle relation of the rear sights. 
In the paragraphs below Capt. Bob Peasley discusses the method of using a thermometer target to determine if this is the case with your sights and what to do about it.
A 100 yd thermometer target would have a 6 in black "bulb" to aim at and then a vertical line running up from that about 2 ft-- assuming you hit the 'bulb" with your 100 yd zeros, then, moving your sights up the usuall 14 min of angle, your hits should fall on the plumb line  14 inches above the first group.
Capt. Bob........

"Kurt:
You may have a better idea or plan but for what it's worth I am passing this along. I have noticed that many of the SR guys are off by as much as 1 or even 2 minutes on their wind zeros. With no sighters this is a big deal as you know. Here is how I obtain a really solid wind zero at 2, 3 & 600.
         I use a thermometer type target at 100 yards and shoot the normal positions. I drop a chalk line from the top right thru the wind line on the bullseye. I put on what I think is my 200 elevation and no wind zero and shoot a 4-5 shot group to see if it is on the vertical wind line. Then put on your 300 elevation and shoot 4-5 more to see where the group is. Then repeat with your 600 elevation. If the 5 shot group is not on the wind line at the 600 elevation start cranking till your group is dead nuts on. Note that setting on the wind knob and mark it. Simple as that, and maybe the guys are doing this or have something even better. They may find that with the 300 elevation they have to put on a couple clicks of wind different that 200 either way to get on the line. Same at 600. Come up from 3 to 600 setting and the group may not follow the vertical line.
        On of my Ar's my wind zero is about 1/2 or 3/4 minute right of the index at 300 and 1/4 or 1/2 minute left of the index at 200. Most likely a position thing but at least I know.
       It's probably not an issue on the AR due to sight design but using this technique I discovered a couple of my M1A's had curved rear sight bases. When it was cranked up it also moved right. So when I went from sitting to 300 I had to go 1 minute left for no wind zero. From 3 to 600 I had to crank on another 2 1/2 or 3 minutes. My first theory was that a wind from the left affected a left hand shooter more than a right handed shooter! Ahhh to be young and foolish again!
      Anyway, feel free to pass this along or not as you see fit. My goal will to be nothing worse than a 9 for wind on everybody's first shot at 600."
Bob Peasley

Thanks Bob