I am not trying to teach you how to suck eggs, so if this is too basic, I apologise.
Remember that a fired bullet must follow an arc-like trajectory to mid-range targets. The distance you zero your weapon will affect where the bullet is in relation to the sights at certain distances in it’s path.
The average line of sight over bore for an AR is 2.6 inches. This means that with a zeroed sight the bullet will begin it’s path 2.6 inches below the dot, then travel upward* until it hits the dot (initial point), continue upward until it reaches the apex of it’s trajectory (max height or mid-range height, also incorrectly called max ordinate- which really applies to a max range trajectory), and then drop until it reaches the true zero point (assuming that the shooter is zeroing at actual distance, not the initial point).
*-by upward I do not mean to imply that there is some magic force that lifts the bullet, the barrel is angled upward in relation to the sights, which causes the bullet’s path to be in an “upward” direction. Technically the bullet is constantly falling away from the bore line.
What this basically means is that there are only 2 distances at which point of aim (POA) will be point of impact (POI)- the initial point and the zeroed distance. All other distances will see the POI higher or lower than the POA, though it may be very small indeed.
The times that hold-over is critical is at close range (0-25 yards) and long range (past the zero distance). The mid range stuff isn’t too difficult, as a 200 yard zero keeps the bullet within 2" of LOS (theoretically hitting the dot) from 10 yards to about 230 yards (M4A1, Mk262). Distances at which this occurs will vary according to ammunition, velocity, and LOS over bore.
There are some really good ballistic calculators to determine trajectory. I personally prefer the JBM Ballistic Calculator. The issue is that you will need to know your Ballistic Coefficient and muzzle velocity. The BC isn’t too difficult to find, but muzzle velocity really needs to be determined by you with a chronograph. Published MVs tend to be a little generous.
The other problem is that these numbers are just numbers, not reality or a substitute for experience. They are a guide only. You must shoot your weapon at actual distance to determine exactly where you hit with your weapon with your ammo. Considering that average shooter/ammo/weapon combinations will provide about 3-4 MOA, predicting exactly where the bullet will land for a single shot becomes a little fuzzy.
What it boils down to is that short of the initial point and past the zero point you will need to hold over the target to some degree to achieve consistent hits, and between those two points you may need to hold under, depending on your precision needs.
The good thing is that you have a range that permits zeroing at distance instead of being forced to zero at the initial point as many are.
Apologies for the lengthy post, hope it helps.
