There are three most popular ways to zero the AR15s and similar 5.56 carbines. These three are a 100 yard zero, a 50/200 zero, and the military 300m zero. Here I am going to talk about the 300m zero.
Unfortunately most people do not have easy access to a 300m range. Even if they did, it makes sense to start at a closer distance. Later on I will reiterate that with a demonstration.
Why the 300m zero? I personally wouldn’t recommend it to most people unless they are already familiar with it from time in the service. The 300m zero has the round first cross the point of aim at 25 meters, then it raises to about 7 inches over the point of aim at about 173 yards, then it is on at 300m. At 400 meters you are about 10 inches low. Still easily on target. Might be easier if I made this a chart.
Chart made from info gathered using JBM ballistic calculator. Figuring firing M855 from a carbine at 2970 FPS at the muzzle and a sight height of 2.6 inches over the bore.
So the real benefit to a 300m zero is that it is easier to use it to hit a man sized target at 400 and 500m just by aiming a little higher. If you are not actually expecting to shoot those distances, something like the 100 yard or 50/200 zero would likely be the better choice.
The Army used to teach to adjust the point of impact (POI) to hit right at point of aim (POA) at 25 meters. Some years ago they realized it was better to have the troops adjust the POI to be about 1/3 of an inch low at 25m to get closer to a correct 300m zero. At reduced ranges small amounts of error will add up greatly at longer ranges.
So what if we are not using a 25m range, but instead the more common 25 yard range? We can see from the chart above that that we want to be 0.4 inches low if we shoot at 25 yards.
Now to get down to the shooting.
If we are using a sight with ranging settings, we want to set it for 300 meters.
In this case I used a Matech rear sight. I set it to the 300m setting. I also flipped it up for the shooting. Using a target at 25 yards, I fired a well aimed group of 3 shots. Why 3? It lets you use the average of the three shots to minimize error. If you had a rifle and ammunition combination that you are extremely consistent with, you could make zeroing adjustment off a single shot. But for stuff like this it is better to shoot groups. The more shots the better, but 3 tends to be the minimum.
I know, from experience, that the M855 ammo I have tends to be about 2 MOA ammo. That means at 25 yards I should be getting half inch groups. If the group is larger than 1/2 inch, I am not doing my part.
That group is most certainly larger than 1/2 inch, so I wasn’t doing my part well there. But it gives me something to work with. I see that this rifle is shooting 4.5 inches low and 1.3 inches right at 25 yards.
Had I started at 100 yards, I would have been impacting 18 inches low. I would have been completely off the target. That is why it makes sense to start up close.
Also note my high quality custom BZO target (black Sharpie on paper). I wanted to demonstrate you do not need a fancy target for zeroing.
On the AR15 carbine, adjustment of the front sight are about 1.75 MOA per click (1/4 rotation), and wind-age is about 3/4 MOA.
I needed to go 18 minutes up, so I decided to make an adjustment of 10 clicks. I also needed to go about 5 minutes left, so I choose to go 6 clicks left. (In hindsight, the math says I should have done 7) Then I fire another group.
After firing another 3 well aimed shots I find another group that is less then perfect, but still gives me good information. This three shot group is 1 inch low from point of aim and half an inch right. So I need to make another adjustment.
Don’t forget, I want the impacts to be a half inch low (0.4 actually). So I want to dial up 1/2 inch (1 click) and left 1/2 inch (3 clicks). I make the adjustment, and fire a new group on a new clean point of aim.
There we go, zeroed in 9 shots.
Now, ideally, you tweak and confirm the zero at the full distance.