"Ideally, you want to mount the scope, as close to the top of the barrel as possible. The distance between the line of the bore, and the line of sight, is called paralax. The greater this distance, the more error tolerance occurs. With most sporting rifles, the scope mounts ontop of the reciever, however with the AR series, often the scope mounts ontop of the carry handle. This gives you about a 3 1/2 in paralax to deal with. You can use this to your advantage to set up a battlefield 0. If you set your 0 to 100 meters, the bullet roughly takes the following flight course. The bullet hits 3 1/2 low at 25 meters, begining to climb to the first 0 at 100 meters, the line of flight crosses the line of sight. The bullet now climbs to 3 1/2 inches above line of sight, settleing down to 0 again around 300 meters, and falling to 3 1/2 in below line of sight by about 400 meters.

In a nutshell, with this 0, you can hold on to center of mass, and it's a hit (within 3 1/2" or less) clear out to 400 meters. With a graduated scope, you can move your reticle mark for an even tighter hit.
Hope this helps, good luck on Z day"

"http://www.zombiesurvivalwiki.com/thread/2696357/Rifle+Zeroing+101
This is my zero write up from '09.

If your sights are 3 1/2 inches above the bore, and you zero for 100 yards, your zero is going to be closer to 150 to maybe 200 (With an AR that you mentioned). The bullet has to climb a total of around 6 1/4" to be on at 300.

If you are using the crap iron sight mount your parallax is actually closer to probably 5". The iron sights alone almost sit close to 3" above the bore, add a big block on top of the iron sights and then a scope on top of that and you are probably pushing 5", which in theory would be close to a 300m zero from 100.

Also, how do you figure that if your sights are 3 1/2" above the bore, you have a 100m zero, that you would hit 3 1/2" low at 25m??? Just think about it.

I'm not trying to rip your post apart, but I just want to make sure you have your numbers straight before someone follows your post and gets completely lost with their zero."

"Keep in mind I'm not going to have a 3.5 paralax though, I mean if I do follow this"

"What about when you guys used the M16, A1 and A2? That's what he's saying"

"M16A1, 2, and 3 then you use the irons. Putting that piece of crap mount up on top the carry handle is about the worst thing anyone can ever do to an AR. It's not solid, it just sits in the groove held down by a dinky little bolt. It's impossible to get any kind of cheek weld and the sights are so high off the bore good luck hitting anything other than at the range zeroed at. I think they had a scope that fit in there back in the Vietnam days, but then someone finally came to their senses and realized that a scope sitting a foot above the barrel (not really, but you get the point) isn't the best idea.

Like cas13f says, you are 1000x better off to plan ahead and get the A4/flat top upper if you plan on using ANY kind of optics.

There is nothing wrong with A1 or A2 irons, but damn it's ghetto to put an optic up there. Especially some giant wal-mart scope.

If your optic is not on the same plane as your iron sights then the gun is going to shoot completely different at various ranges, there's just nothing good to say about the "carry handle" scope mounts.

Hell, you can buy just the flat top upper for about $90 and swap it out yourself with a few tools."