NOTE - This is an advanced system and the reader should have a solid grasp on how a wind rose works and the Gun MPH system.
The gun mph system is the fastest way to dope our rifle for wind. It is easy, intuitive, and very quick once we get the hang of it. It’s great for making an initial wind call and for reverse calculating what the actual wind speed is. (If you are not familiar with the Gun MPH system and what its wind brackets are, I suggest reading the Managing the Wind article about this system.) If we miss our first shot from a bad wind call, then make a correction and hit our target, we can reverse calculate to know what the cross wind speed and the actual wind speed are. Here is an example of that process.
Example 1:
Target at 400yds.
Initial wind call is 0.8 mil right for the wind we measured (15mph at 1:00) for our 5mph gun.
First shot = miss off right edge (too much wind hold).
We observe the miss splash and see it is 0.4 right of the target center.
Move to 0.4mil right to center and send the 2nd round - Impact on center.
Reverse calculate the wind bracket - 0.4mil/0.4mil = 1.0 bracket.
Reverse calculate the wind mph - 1.0 x 5mph = 5mph cross wind.
From a wind rose: A 5mph cross wind correlates to a 10mph actual wind speed at 1:00.
Portion of Corrected Wind Speed for Angle wind rose
We can easily reverse calculate which bracket the cross wind speed falls into (1.0), then reverse calculate to the cross wind speed (5mph), and from this we can work the wind rose to get the actual wind speed (10mph at 1:00). But, in a shooting situation, if our next target is the same firing direction, we are probably not going to calculate back to the actual wind speed or even to the crosswind speed to engage our next target. We’ll most likely reverse calculate to the wind bracket, then range the next target and make our wind call for the same bracket, as shown in Example 2 below.
Example 2:
Target 2 is 700yds in the same direction.
From Example 1, the wind speed is 1.0 bracket.
Gun MPH wind hold - 0.7mil x 1.0 = 0.7mil hold à Impact.
This system is very easy to use when we have multiple targets in the same direction. But what do we do if we need to shoot multiple targets that are not in the same direction? We’ll need to reverse calculate all the way to the actual wind speed and direction (10mph at 1:00) to readdress the next target with a proper wind, as shown in Example 3a below.
Example 3a:
From Example 1, the wind is 10mph at 1:00. (Reverse calculated bracket to cross wind to actual wind)
Target 3 is at 400yds and 30 degrees left of the Target 1.
Wind speed and new direction = 10mph at 2:00.
Use a wind rose to get a cross wind speed - the cross wind is 9mph.
Gun MPH Bracket - 9mph/5 = 1.8 bracket
Gun MPH wind hold - 0.4mil x 1.8 = 0.7mil wind hold
We had to reverse calculate from the first target all the way to the actual wind speed and direction, then use this speed at the new direction in the wind rose to get the cross wind speed, then calculate the gun mph bracket, … to finally calculate the gun mph wind hold. This took a lot of steps to calculate our way out of one wind call and into the next wind call.
How can we do this faster? How can we do this without having to calculate back to the actual wind speed and direction to move forward onto the next target? If we write out the wind rose as a function of the wind brackets, we can change direction without caring what the cross wind or actual wind speeds are.
This is the Gun MPH Wind Bracket Rose.
Let’s work through the same Example 3a again and we’ll see how we only need the Gun MPH Wind Bracket Rose to get a wind call for the Target 3.
Example 3b:
From Example 1, Target 1, we know we are in the 1.0 bracket and the wind direction is 1:00.
Target 3 is at 400yds with a wind direction of 2:00.
Use the Gun MPH wind rose, locate the 1.0 bracket at 1:00 (2nd ring from center).
Move around the circle along this same ring to the 2:00 wind direction and read 1.7 (2nd ring from center).
Gun MPH wind hold - 0.4 x 1.7 = 0.7mil hold.
Portion of Gun MPH Wind Bracket Rose
We arrive at the same answer but with half the steps required. After the first target we didn’t have to reverse calculate the cross wind speed or the actual wind speed. We simply divided our wind hold on the target by the gun mph hold (0.4mil/0.4mil = 1.0 bracket). Moving into the next target we didn’t have to take the wind speed from target 1 into a wind rose to get the cross wind speed or calculate a wind bracket. We only had to multiply the new bracket number, from the Gun mph Wind Bracket Rose and multiply this by the Gun MPH value for that target (0.4 x 1.7 = 0.7mil).
Here are the direct steps from target 1 to target 2, 3, or 4. (Values from the Examples above are included)
Impact Target 1.
Calculate this wind bracket. Divide the wind hold by the Gun MPH value at that distance. (0.4mil hold / 0.4mil = 1.0 bracket)
Evaluate the wind direction for Target 2.
Find this bracket value from Target 1 in the Gun MPH Wind Bracket Rose (1.0) at the wind direction for target 1 (1:00).
Move around the circle to the new wind direction to read the new Gun MPH Wind Bracket (1.0 @ 1:00 to 1.7 @ 2:30).
Calculate the wind hold with the Gun MPH (0.4 x 1.7 = 0.7mil).
Here is another example with some added notes about the process on the rifle.
Example 4:
Target 1 is at 400yds with a 1:00 wind.
Impact Target 1 (0.4mil Hold).
Calculate Target 1 wind bracket - 0.4mil/0.4mil = 1.0 bracket.
Target 2 is right of Target 1 at 700yds. (Angle not given because we cannot measure that in the field.)
Point the rifle to Target 3 and position our body square to the rifle and target.
Evaluate the wind direction by feel and vegetation - 10:30 direction.
Read the Gun MPH Wind Bracket Rose from 1.0 at 1:00 to the same ring at 10:30 à 1.4 bracket.
Gun MPH system - 0.7mil hold x 1.4 bracket = 1.0mil hold (left). (Mentally I would use a 1.5 bracket and get a 1.1mil hold because I always like to round my wind hold up.)
Engage target - Impact.
Portion of Gun MPH Wind Bracket Rose
Using the Gun MPH Wind Bracket Rose, we can easily transition to different targets in other directions and still quickly calculate a representative wind hold from the data on our first target. No need to start your wind calling process over and guess at the wind hold anymore. This rose and a little mental math will give you a very good starting point with data already collected. Don’t forget the final wind call will be affected by terrain features, which we should be looking for. This type of wind rose mounted on the rifle during a “Find it - Range it - Engage it” style match could prove very useful where wind calls need to be done quickly.
Initial Wind Call
What about the initial wind call? Can the Gun MPH Wind Bracket Rose be used to calculate the initial wind call? Yes. If we replace the 12:00, 3:00, 6:00 and 9:00 bracket values (1,2,3,4,5) with the wind speed for our rifle brackets, we can use it for the initial wind call. (5mph gun = 5,10,15,20,25) (4mph gun = 4,8,12,16,20) Example 5 below will show how this can be used to get an initial wind call.
Example 5:
Target at 400yds
Measured wind speed and direction - 15mph at 1:30.
Use the 15mph ring (3rd ring from center) and move to the 1:30 direction to read 2.1 bracket
Gun MPH wind hold - 0.4mil x 2.1 = 0.9mils.
Gun MPH Wind Bracket Rose
To use the Gun MPH Wind Bracket Rose for the initial wind call, we ‘ll need to have our Gun MPH bracket speeds in the four cardinal direction slots. Below are the Roses for 4,5,6, and 7mph guns. Below is also a blank rose to print it and write your own wind bracket speeds into it, if needed (for those that need m/s…). PDFs are below these images.
