The instructions for my Colt 6520 say to raise the rear sight up one notch from 8/3 when zeroing, but than lower it back down to 8/3 when I am done. That does not make sense to me. How is it that I can adjust the sight after it has been zeroed but it is somehow "zeroed" in both positions. I'm confused. If my question needs clarification please let me know.

If I'm not mistaken. , and I'm remembering this from back in basic training since we were zeroing on paper at 25m with a 300m target represented  it had to do with the rounds path of travel hitting higher on the paper than it would if we were actually hitting a target at 300 meters. So after zeroing on paper  its assumed your next shots will be at the qual range at known distance pop ups , so then we would turn it back down from 8/3 +1

Thanks. That is an interesting explanation. So turning back down from 8/3 +1 to 8/3 after zeroing keeps your zero at 25/300 yards? Why does it hit higher on paper than a real target I wonder? I would love to hear more about it if anyone knows.

Think of of it like zeroing any other optic. You zero at a known distance, then adjust accordingly for the extended range beyond that zero, but that puts your sights at zero+adjustments. In this case, you're zeroing at 25M, but intend to shoot at up too 300M. But, instead of zeroing, then adding elevation to the sights, you're taking away elevation to start, then adding that elevation back in for a proper zero as if you were originally zeroing at 300M. Also, it isn't exactly a dead nuts zero. The round is still going to hit a bit high at close range, and a bit low at long range. This systems was designed to hit center mass with no adjustments from 0-300M, and center mass means a pretty good sized area.

And to answer this question, "Why does it hit higher on paper than a real target I wonder?"   The bullet doesn't like to hit people in the head, or "real targets" for that matter, so it purposely shoots low so to not offend the rest of the ammo, or cause unnecessary blood shed. But it absolutely hates paper! Like a cat to water, and will absolutely 'ef that shit up!

There should be a "Z" on the sight drum, turn it to that.  A 25M zero target represents the size of a 250M pop up target or man silhouette.  By adjusting the sight after zero you are elevating the sight to the proper position of what it would need to be zeroed at 250M.  Basically the 25M zero is a faux-zero.  BTW a 36M zero is actually more useful because you can zero without h adjusting the sights afterwards and be only -8 inches or so at 300M.

turning the rear sight past 8/3 compensates for you shooting at 25m. once it's back to 8/3 the bullet will be poa/poi at 36 YARDS / 300 METERS

That rear drum is basically a bullet drop compensator as the A2 has a target/match type rear sight. Zeroing you only adjust the FSP and the windage. The drum compensates for distance.

8/3 is your 300 meter setting. Adding 1 click to it brings it in line with the 25m zero point. Bring it up to 4 and you should be zeroed at 400m. 5 will be 500m. That's meters. You'll have to do some experimenting for translating the meters into yards.

For the rifle I competed with a few weeks ago, 400 yards was "4" - 1 click and 500 yards was "5" - 2 clicks since  yards are shorter than meters and at 500 yards there's a decent amount of difference. Some rifles might need a click or 2 in one way or the other.

http://www.ar15.com/forums/t_3_127/645835_A2_8_3_rear_sight_settings_for_ranges_measured_in_yards.html

OP, if I may. ..... This is of interest to you. adjust for your barrel length and ammo accordingly.

Concur, just shot a match last week with an A2 and I was 4+1 at 400yds using a 36yd zero

The new improved 50/200 yard zero for the M16.

Search for it on the web.........the new way to zero your A2 sights at 50 yards.

Give the guy a break on searching  Improved Sight Setting

Current Army doctrine states to acquire a point of aim point of impact zero at 25M to achieve a point of aim point of impact zero at 300M where the bullet crosses the line of sight again. Note that the bullet impacts .33 inches low at 25M. This is why it is necessary to raise the BDC to 8/3 plus one click during the zeroing process, and then return the BDC to 8/3 for qualification or battle.

US AMU Bullet Path Tables and Diagrams

old way 25M zero.......having alot of trouble shooting high at normal combat ranges........up 200.

Improved Battle Sight Zero

New way 50 Meter zero.......loosen the set screw......turn the drum all the way down .....tighten set screw. Zero at 50 yards.

Better Details on the web sight

this should give you a 50/200 in the ball park.

this was started in the Marine Corp.

I always thought there was a reason for the sight marked 0-2 .......

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Quoted:
turning the rear sight past 8/3 compensates for you shooting at 25m. once it's back to 8/3 the bullet will be poa/poi at 36 YARDS / 300 METERS

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This is the correct answer.  After the transition to the m16a2,  the Army didnt want to build 36yd ranges (like the USMC) when they already had 25m ranges.

Which is why the Marines use a 36 yrd BZO. It's what your gun and sight were designed for, it's the easiest way to do it if you can measure the range to get the target at the proper distance. Never understood why the Army did this. Never understood why they don't train to shoot to 500 either, but it's the military, it's not supposed to make sense.