Wind is almost always the hardest variable for a precision rifleman to account for. The shooter can utilize all of the information that can possibly be acquired about wind direction and speed, but ultimately, that first shot will always be the shooter's best guess. However, after that first shot impacts, whether it was on the target, berm, or downrange, we can make corrections and utilize that information for first-round impacts on other targets. Gun Number is the solution we can use to achieve these results.
Gun Number is the full value wind speed, in miles per hour (MPH), required to push your bullet horizontally off target by 0.1 MIL for every 100 yards.
Table 1: Distance vs. Wind Drift for the Gun Number Solution
Use Gun Number, also known as Quick Wind or Short Wind Formula, to make better opening wind calls and corrections. We can further utilize Gun Number to quickly apply wind holds to stages with multiple target distances, i.e. Troop Lines.
Gun Number is not a one-size-fits-all value. It must be determined for every rifle and load that the shooter employs. In order to determine Gun Number, you'll need a solid and trued ballistic profile built in a solver, like Applied Ballistics, Hornady 4DOF, or other programs.
To Determine Gun Number:
Have a ballistic profile set up in your solver for your rifle and load.
In the solver, set the wind direction to 270° (9 o'clock / full value).
Turn off spin drift (do not use Total Wind).
Set wind speed to the starting value (see Table 2).
Set results table out to 1000 yards in 100 yard increments.
Review wind drift in the solver results to see if your wind speed input closely matches Table 1.
Increase or decrease the wind speed input to find the closest match to Table 1.
The MPH wind speed input that most closely matches Table 1 is the Gun Number for your rifle and load.
Table 2: Common baseline Gun Numbers for carbine shooters.
After the baseline Gun Number is established, double the wind speed value in your solver. Notice that the wind drift also doubles. Now triple the wind speed; the wind drift will triple as well. Use the example in Table 3 for 5.56 shooters:
Table 3: Wind drift example for 5.56 rifles with a 3MPH Gun Number.
In the solver, notice that around 600 yards there is a jump from 0.1 MIL increments to 0.2 MIL. This is because the velocity of the bullet is slower and the projectile is in the air longer, allowing the wind to provide more input. The solver's solution will be more accurate, but Gun Number is intended to be quick, simple math that you can do in your head and on the fly. If you're looking for a more accurate solution, use a solver, but referencing a solver in the middle of a stage will not always be possible. This is where the simplicity of the Gun Number solution comes into play.
Applying Gun Number to Multiple Targets
Gun Number can be applied to targets at multiple distances. Once the first shot establishes wind speed, use the known wind drift of the bullet to back calculate full value wind speed. Then you can simply apply that wind speed to your Gun Number solution for all remaining targets, provided that the targets are all in the same direction.
For example, if you have a 3 MPH gun and your first shot shows you need to hold 1.0 MIL of wind for a target at 500 yards, then the full value wind speed is 6 MPH (Wind drift at 500 yards should be 0.5 MIL at 3 MPH, so it will be 1.0 MIL at 6 MPH). If the next target is at 700 yards, you can then apply a wind hold of 1.4 MIL (Wind drift at 700 yards should be 0.7 MIL at 3 MPH, so it will be 1.4 MIL at 6 MPH).
Gun Number can also be used to bracket wind on stages with targets at multiple distances. Using one of our 3x5" Data Cards, shooters can build out multiple wind calls on the right hand columns. The middle column, Wind 2, is used as your best guess wind speed. Wind 1 and Wind 3 are wind speeds above and below your best guess. When your first shot establishes the true wind speed, you can check your table and determine which column best represents the wind hold. From there, you can use that column for the remaining targets on the stage.
Finally, the simplicity of the Gun Number table uses 0.1 MIL of wind drift for every 100 yards of distance. This is very simple to remember, but what about targets that are in between the 100-yard increments? The shooter can choose to use the wind drift that relates to the 100-yard increment, or the shooter can choose to round up to the next increment if it makes sense. For example, if the target is at 820 yards, you'd likely use the 0.8 MIL adjustment. However, if the target is at 870 yards, you may also consider rounding up and using the 0.9 MIL adjustment. This is the shooter's choice based on preference and information that is available.
Final Notes on Gun Number:
Gun Number only applies to the full value component of the wind (90° left or right). For example, if the wind is blowing at a true velocity of 10 MPH but only affects the bullet at half value (coming at or away from the shooter at 30°), then we need to use the 90° full value component of the wind, which is 5 MPH. Only apply the wind speed value that will affect the bullet's flight to the left or right when using Gun Number math.
Gun Number can change at different elevations! The change in air pressures at differing elevations can have an effect on Gun Number. Your 3 MPH gun at sea level may become a 4 MPH at 4,000 feet! When heading to a different range, recheck your rifle's Gun Number during the zero and data gathering process.
Many shooters may reference that the wind dots in a Tremor 3 reticle do the same thing. This is correct that Gun Number and Tremor 3 are very similar, however, some users have noted that their solver's Gun Number and the Tremor 3 wind dots were slightly different (e.g. the solver shows a load to be 5 MPH but the Tremor 3 shows it to be a 4 MPH dot). In this case, it is likely best to use the Tremor 3 as long as the user understands the wind dots.
Gun Number is a close approximation designed to be quick and simple, but will not be as accurate as a solver. If you're shooting in winds that are 4-5 times greater than your Gun Number, it might be a better choice to reference the solver's solution.
Do you use Gun Number on the range or in the field? Let us know in the comments!