This post is going to be deliberately simplistic and lacking in ballistic charts. The purpose is just to advocate to you the benefits, in my opinion, of the 100 meter (or yard if you insist, as it does not make a world of difference in practical terms) zero for a 5.56 AR style platform.
I am a great fan of the 100 meter zero. My reasons, in simple terms, are this:
Ballistically, if you zero at 100 meters, there is not a great deal (a couple inches) difference in your point of aim and point of impact between 0 and 100 meters. Due to the ballistics of the 5.56 round, and this will vary very slightly due to factors such as barrel length and the actual round you are firing, the round will ‘graze through’ at around 100 meters and drop thereafter.
Why is this good? It means that I know I am good from 0-100 meters. This is my most likely firefight range, perhaps out to 200 meters. Beyond 100 meters the round is dropping in relation to the correct impact point in the center of my target. This means that as ranges increase, I simply have to adjust my point of aim upwards to compensate. If I forget, then my round will strike low, which in my opinion is preferable to striking high, because I have more chance of seeing the strike and applying my fire by raising my point of aim.
Also, I am a fan of the Trijicon ACOG x 4. With a 100 meter zero in this sight, I can use the range finder and graduated ranged aiming points to accurately shoot beyond 100 meters. I don’t have to adjust sights like I would have done when using adjustable iron sights (setting for range), I just have to line up the correct aiming point for the range, effectively raising my barrel to compensate for increased ranges.
If I am forced to zero at 25 meters, I will aim two inches below the correct impact point (center of target) but zero to the center of the target. This compensates for the ballistics of the rounds striking a little low out to 100 meters and in effect gives me a poor mans 100 meter zero.
As soon as a round exits the barrel gravity begins to work on it and it will drop towards the ground. If your barrel is parallel to the ground the round is, infinitesimally initially, dropping as it exits the barrel. The only reason that a round goes ‘up’ as it exits the barrel is because your sights are set to raise the barrel slightly so the barrel is not parallel with the ground. So, in simple terms, for zeros at ranges less then 100 meters you will have two points where the round is ‘zeroed’ or will strike center of target for the same point of aim. That will be the initial zero range, which may be for example 25 meters, as the round is still rising, and the second time it goes through center when the round comes down on its downward fall. For a 25 meter zero that may be around the 300 meter mark. This is why people talk about ‘near’ and ‘far’ zeros.
For a 100 meter zero there is no near and far zero, there is just the 100 meter zero. The round rises slightly (within a couple of inches) from 0 to 100 meters. Then it grazes through the center of the target at around 100 meters (anywhere from around 90 meters to 120 meters, ish) after which it is dropping until eventually it will hit the ground.
If you zero at 100 meters, at longer ranges you are just raising your barrel/sights to compensate and if you don’t your round will strike into the dirt in front of the target. If you forget, or don’t know about application of fire and adjusting your aim upwards, if the enemy is advancing, at least they will advance ‘into your zero.’
If you zero at 25, or 36 meters, or whatever, then your round is still rising after that until it begins to fall and come back down through center anywhere from 300-ish meters. In that case, you are not sure if you need to raise or lower your point of aim at various ranges You may end up shooting high. I would rather shoot low than high, because then I can adjust upwards and walk my rounds onto the enemy.
Live Hard, Die Free.