I love long range shooting because I love numbers. I love ranging targets based on my estimates of their size, I love experimenting with different overall cartridge lengths and chasing perfect concentricity well beyond the point of diminishing returns, because I just think it’s fun.
When time pressure gets involved, sometimes you need to be able to play with numbers in your head without coming off the rifle and fiddling with a Kestrel or other calculator.
Finding your Gun Number simplifies your wind calls, enabling quick adjustments based on wind speed, distance, and your rifle’s ballistics. This guide will walk you through determining your Gun Number and help you apply it in the field.
What is a Gun Number?
A Gun Number is the full-value wind speed (in miles per hour) that will push your bullet horizontally 0.1 MIL off target for every 100 meters of distance. It’s a simplified figure that helps determine how much wind correction is required based on your wind estimate, typically calibrated around 500 meters where you want to see 0.5 MILs of drift at your gun number wind speed.
Think of it as your rifle and load’s unique fingerprint for wind drift. Once you calculate your Gun Number, you can use it to quickly scale your wind adjustments across various wind speeds and distances without needing to reference ballistic calculators or dope cards in the moment.
For example, if you have a 4 mph gun number, a 4 mph full value wind will drift your bullet:
- 0.1 MIL at 100m
- 0.2 MIL at 200m
- 0.3 MIL at 300m
- 0.4 MIL at 400m
- 0.5 MIL at 500m
- 0.6 MIL at 600m
The beauty of this system lies in its simplicity. Inside 500 meters, your wind call for wind speed matching your gun number is 1/1000th the distance to the target. Double the wind speed? Double the drift, and so on.
Why Gun Number Matters
During competition or field shooting, you don’t always have time to pull out your ballistic calculator and reference detailed wind tables. Gun number gives you a mental framework for making rapid corrections based on what you see downrange. After your first shot reveals the actual wind conditions, you can quickly adjust for subsequent targets at different distances without needing to consult charts or apps.
This becomes especially valuable when engaging multiple targets at varying distances, such as in troop line scenarios. Your first round impact tells you the real wind speed, and from there you can rapidly calculate holds for all remaining targets.
The purpose of this system is quick, rough-and-ready estimation. In a match, you’ve got some room for error based on target size because you’re looking for an impact anywhere on the target, and not necessarily in what would be the killzone on a living animal.
Step 1: Gather Necessary Data
Before calculating your Gun Number, collect the following information about your rifle and ammunition:
- Ballistic Coefficient (BC) of your projectile
- Muzzle Velocity (in meters per second)
- Ballistic calculator (such as Applied Ballistics, Hornady 4DOF, or a Kestrel device)
These details will allow you to determine wind drift across various distances. You should have DOPE for your rifle to verify this information. If you don’t, get out to the range and collect some data first. If you’re just working with hypotheticals, you can use the published data for factory ammo to get these numbers.
Step 2: Calculate Wind Drift at 500 Meters
For the most accurate determination of your gun number, follow this process:
For this example, we’re using a 175gn Sierra MatchKing traveling at 2550fps.
- Input your rifle’s profile into a ballistic calculator. If you don’t have a Kestrel (I can help) or phone app, head over to JBM Ballistics and use the calculator there.
- Enter a full-value wind speed (wind blowing from 3 or 9 o’clock) and compute the required wind hold at 500 meters. Adjust the wind speed in the calculator until you achieve a wind correction of 0.5 mils at 500 meters. Eventually, you want to see a ballistics table that looks something like this:
| Distance (m) | Windage (MIL) |
|---|---|
| 100 | 0.0 |
| 200 | 0.1 |
| 300 | 0.2 |
| 400 | 0.3 |
| 500 | 0.5 |
| 600 | 0.6 |
| 700 | 0.8 |
| 800 | 1.0 |
| 900 | 1.2 |
| 1000 | 1.4 |
- The mph wind speed input that most closely matches this pattern is your Gun Number.
Important Note:
As you might have noticed, this renders an easy to remember adjustment inside 500 meters, where your wind call for wind speed matching your gun number is 1/1000 the distance to the target ±.1 mils.
For example, if you’ve determined your have a 4 mph gun, then a 4 mph wind gives you a drift of 0.5 mils at 500 meters. An 8 mph wind then gives you 1 mil of adjustment at 500 meters, and a 12 mph wind gives you a 1.5 mil adjustment.
Beyond 600m, the drift values start to increase more rapidly. This occurs because your bullet has slowed down significantly and spends more time in the air, allowing wind to have greater effect. Less efficient bullets (lower BC) may need an additional 0.1 MIL added around 500-700m, while high BC bullets with high muzzle velocities might need 0.1 MIL subtracted at mid-range. Don’t expect perfect alignment at every distance, but the overall pattern should be close.
Step 3: Practical Application of the Gun Number
Once you’ve established your gun number, the math becomes straightforward. Let’s walk through some real-world examples.
This guide assumes you have some practice estimating wind speed through observation of environmental effects.
Basic Wind Holds
Suppose you have a 4 mph gun and a full value wind of 8mph, and you’re engaging a target at 500m. As your wind drift at 500m should be 0.5 MIL at 4 mph, it will be 1.0 MIL at 8 mph. If the next target is at 700m, you can apply a wind hold of approximately 1.6 MIL (wind drift at 700m should be 0.8 MIL at 4 mph, so it will be 1.6 MIL at 8 mph).
Angled Wind Example
Let’s assume you’re looking at what you believe is a 9 mph wind through the optic of your 4 mph gun. The wind is quartering at you. You’ve ranged your target at 600m.
At 45°, a quartering wind is 3/4 value, (mathematically it’s closer to 70%, but most people can handle mentally solving quarters more easilly). 3/4 of your 9mph wind is effectively a 6.75mph wind, but you round down to 6mph for simplicity, and because you know you’re fudging a bit high anyway.
From there, you add 50% (the extra 2mph above your 4mph gun number) to your 600m value and get .9 MILs.
Let’s run the real numbers.
Assuming you correctly estimated the windspeed at 9 mph, and the quartering angle of the wind is in fact 45°, the true cosine value of the 9 mph wind is 6.36 mph. Plugging that into our calculator, we get an adjustment at 600m of 1 MIL.
A full size IPSC target at 600m is approximately .75 MILs wide, so with a center hold, the .1 MIL error is trivial. In a few seconds you’re only .1 off the actual value and you figured it in your head without taking your eyes off the target with some quick and simple math.
It’s far easier to get off target with a bad initial wind estimate than by simply roughing up the numbers a bit for simplicity.
Suppose that 9mph wind is actually a 12mph wind. Now the actual adjustment value is for a 8.49mph effective wind. Your measured wind call should now be approximately 1.3 MILs but you’re still adjusting .9 MILs based on your incorrect wind estimate. A bad wind call is far more likely to cause problems than some generous rounding of values to simplify your math.
You might still get an impact on a full size IPSC target in the above example. But would you accept a deviation of that size on an animal you’re looking to take ethically?
Ask yourself how confidently you can see the difference between a 9mph and a 12mph wind or accurately determine the angle the wind is coming from to get the correct wind value cosine.
Making Corrections Between Targets
Suppose, for example, shooting your 4 mph gun your initial wind call was 0.5 MIL for a 600m target, but you hit 1.0 MIL left of center:
Your impact reveals you needed 1.5 MIL total hold (0.5 + 1.0). At 600m with your 4 mph gun, the base drift should be 0.6 MIL. Your actual needed hold of 1.5 MIL tells you the real wind is 10 mph (1.5 ÷ 0.6 = 2.5, and 2.5 × 4 = 10 mph).
Now your second target appears at 850m. Using your gun number pattern, 850m needs approximately 1.1 MIL drift at 4 mph (interpolating between the 800m and 900m values). With the actual wind at 10 mph (which is 2.5 times your gun number), your hold should be approximately 1.1 × 2.5 = 2.75 MILs.
Critical Considerations
Full Value Wind Only
Gun number uses full value (90-degree) wind. If wind is coming from an angle, you need to calculate the effective full value component first.
Common angle values to remember:
- 90° (direct crosswind): 100% value
- 60°: approximately 90% value
- 45° (quartering): approximately 70% value (or 3/4 for quick math)
- 30°: approximately 50% value
Example: A 10 mph wind at 45 degrees (quartering) only has about 7 mph of full value effect. You would then use that 7 mph figure in your gun number calculations.
Elevation Changes Matter
Your gun number can change at different elevations due to air density differences. A load that’s a 4 mph gun at sea level might become a 5 mph gun at 2000m elevation. Always verify your gun number when shooting at a different elevation from your typical range.
It’s an Approximation
Gun number is designed for speed, not absolute precision. In extreme wind conditions (4-5 times your gun number or higher), or for critical shots on live game at long distance, take the time to use your full ballistic solution. Gun number shines in competition environments where speed matters and targets are reasonably sized.
Real-world testing across multiple cartridges from .223 to .338 Lapua has shown that inside 1000m, gun number calculations are typically within 0.1 MIL of actual ballistic calculator solutions—and 0.1 MRAD of deviation at 500m is only 5cm, and 10cm at 1000m. For field-expedient math, that’s remarkably close.
Bracketing Technique
For stages with multiple targets, you can prepare three wind columns on your data card: your best guess wind speed, and then one column each for higher and lower wind speeds. After your first shot reveals which column is closest to actual conditions, apply that column to all remaining targets.
Practical Training and Validation
The best way to internalize your gun number is through repetition. Practice the mental math at home: pick random distances and wind speeds, then calculate the hold. Time yourself. The goal is to make this second nature.
During practice sessions, intentionally vary your wind calls to see how your gun number predictions compare to actual impacts. This builds confidence in the system and helps you understand its limitations.
Practice making these rough estimates and compare them with your Kestrel at the range. As with all things wind related, it’s harder to make a precise science of this because you can’t measure the conditions along the entire flight path. Only consistent trial and error can help you master these skills through downrange validation.
Many shooters also write their gun number and a quick reference table on their arm board or data card for easy access during matches. Even though the goal is to memorize the pattern, having that backup reference can prevent errors when you’re under stress.
Conclusion
Your gun number transforms wind calling from a complicated lookup process into simple arithmetic. While it won’t replace a full ballistic solution for every situation, it provides the speed and simplicity needed for dynamic shooting scenarios.
The real challenge isn’t the math, it’s accurately estimating wind speed and direction. Gun number simply makes the calculation portion instantaneous once you have that estimate. A perfect calculation with a bad wind read will miss the target, while a slightly rough calculation with a good wind read will get hits.
Take the time to determine your gun number, practice the mental calculations, and validate your estimates against your Kestrel and actual impacts. You’ll find yourself making confident wind calls even as the clock counts down, without ever taking your eye off the target.