There are two methods for handling the rifle zero and the zero angle calculation.
The first is familiar, the second is what we really want. Let's take a few minutes to break down the differences and best practices for precise shooting.
In the Zero Range method, the solver establishes the baseline or starting conditions and the solver drives the rifle setup. The solver will calculate the ballistics with a starting condition called Zero Range. This starting condition is defined by a zero range, and an elevation and/or windage POI Offset. The calculated ballistics assume that the rifle matches the baseline for the conditions present when making the calculations. This method does not correct for zeroing conditions.
Let me explain, we’ve all set up a target and zeroed a rifle. The distance to the target is called Zero Range. Hopefully you’ve successfully reset your turrets, so your adjustments are set to zero. Usually, if our group is really close, we just accept that as good enough and go shooting. The small amount of difference between your measured group center and the center of your aim point is called POI offset. This can be small or rather large. For example, it is very common for a rifle zero to be set at 100 yards, and be one inch high. In this example, the zero range is 100, the adjustment is zero, and the POI Offset is +1.0 (a “-” sign would be used if the impacts were low).
We’ve covered the nomenclature and a typical scenario for using Zero Range style of inputs for a ballistic program. With this method, all of your ballistic calculations will be made from this fixed set of zeroing conditions. In general, this simple method is very easy to understand and use. However, as our skill and equipment develops, we can start to see some cracks in this methodology.
Here’s the first problem. If I go to a location with environmental conditions and sighting conditions that are significantly different than where the initial zero was set, I will have to set up a target and check (and likely adjust) my rifle zero. We commonly zero our rifles to be right on at 200 yards. In an extreme environmental shift, I’ve had to adjust my zero a half MOA (1 inch at 200 yards). I don’t have a problem checking my gun after a long travel trip, but what about an early drive up the mountain? What about an extreme backpack hunt where you might see a significant amount of effect in one day’s worth of hiking?
Several ballistic apps have started to allow users to enter the conditions that they encountered when zeroing the rifle. Once the data is known, the back end calculations can adjust all of the future ballistic solutions for the zeroing conditions. What is happening here is that the solver is calculating the exact starting angle between the gun (centerline of the barrel, or Departure Angle) and the scope (when set to zero, called Line of Sight).
Some of the more modern ballistic apps call this angle Zero Angle, and solve for it explicitly. If the app is set to use Zero Angle instead of Zero Range, the Zero Angle becomes the driving input. The Revic Ops ballistic app features this calculation and stores it as part of the Profile.
In application, after adjusting the rifle mechanical zero, we put in all of our zeroing conditions and let the solver calculate the “zero angle” related to the fixed mechanical adjustment and the bullet’s ballistic performance for the environmental and shot conditions that existed when the mechanical zero was established. In effect, now the rifle setup drives the ballistic solver. As long as no mechanical adjustments are made to the turret’s zero position, the zero angle value can be used as the baseline for properly calculating the ballistic solution for all conditions without rechecking your zero.
The future of all ballistic apps is to shift to Zero Angle methods to calculate the exact ballistic solution required for our long range shooting.