It sounds complicated. Let me give you a simple example. A scope is like a ruler. Now you have a ruler with markings but no numbers, just like the one below, and you need to measure something.

To measure something, one must first determine a zero point. I can take the beginning as the zero point, or start from the middle, as long as it is within the measurement range.


It's simple, right? Just determine an appropriate zero point, and then you can start measuring. The same goes for the scope. You need to have a zero point first before you can "measure things". However, the zero point of the scope cannot be zero meters, that is, the moment the bullet exits the muzzle cannot be set as the zero point. I'll explain why later.
How a Crosshair Reticle Scope Comes Into Being
First, let's take a look at how a crosshair reticle scope comes into being. First, we have a telescope with no reticle at all, completely blank. Then, we use wire and screws to fix it onto the gun. After that, we draw a dot at the center of the scope's field of view as a reference point.

Then fire a shot at a target 100 meters away and find that there is a deviation between the point of impact and this reference point. Now, it's certain that using the reference point as the aiming point for shooting won't work, because it won't be accurate.

So we made a series of operations, turning various screws on the telescope, adjusting its elevation and deflection, and gradually making the reference point and the point of impact approach and coincide.

Creating the Original Scope
Finally, we got an original scope, an original scope with a zeroing distance of 100 meters. This means that as long as the center point, reference point, and aiming point of the scope are aligned with a target 100 meters away, the bullet will accurately hit that point.

But we can't always shoot at targets exactly 100 meters away. We also need to shoot at targets at other distances. For example, when shooting at a target 300 meters away, the following situation will occur: the point of impact is below the aiming point at 100 meters.

Distance Calibration
In that case, we'll conduct test shots at different distances. Then, based on the impact points, we'll mark a graduation at each distance to aim at targets at different ranges, like this.
The first cross graduation is for aiming at targets from 100 to 200 meters, the second is for 300 meters, the third for 400 meters, and the fourth for 500 meters.

The Role of BF6 Range Finder
Finally, I would like to say that none of us are professional snipers, so we need to use a rangefinder. The BF6 rangefinder can help us quickly calibrate the zero point distance and the target center, helping us hit the enemy faster and more accurately.