Once I reach my Final Firing Position (FFP), I will start gathering environmental data. I will actually start gathering data previous to this (like high and low temperature, elevation, barometric pressure, and forcasted weather). I like to start with the data that is not going to change, such as elevation. I'll then move on to the slower changing data, like temperature, and last is wind (ever changing).
Elevation
To get elevation, I'll refer to a topographical map with elevation, as well as GPS and a Kestrel.
Elevation: 1062 ft
Barometric pressure
I gather barometric pressure information from a barometer, GPS, Kestrel, or a ballistic calculator such as Strelok pro using GPS. Being that the elevation at my FFP is 1062, I can estimate that the barometric pressure will be 28.4 inhg.
Latitude and Longitude
Latitude and longitude is additional information that I will collect before heading to my FFP. This data can be collected using maps or GPS. This information is to adjust for coriolis and spindrift.
Humidity
I typically get humidity from the weather station or a Kestrel with a hygrometer.
Humidity: 50%
Ambient Air Temperature
I use a Kestrel for this, but I also carry a nice compass that has a thermometer.
Air temperature: 88 F°
Bullet Temperature
Bullet temperature (or more accurately, powder temperature) needs to be measured. This will determine the speed of the bullet. I use a laser thermometer
Powder temperature: 88 F°
Distance To Target
The easiest and most effective way to determine the distance to your target is to use a rangefinder. You can also Mil the target if you know the target's size.
Distance: 1400 Yards
Target Size
This will help with hit probability and amount for error. It's also imperative to have this information if you are using the reticle in your scope to Mil the target.
Target Size: 5.9 Ft (69") x (17")
Angle of Fire
The angle of fire will change my elevation adjustment, depending on how steep the angle is. Simply multiple the distance by the angle cosine. This corrected distance will also be used to adjust for coriolis.
Angle of fire: 0
Wind
Estimating wind is always the hardest part for me. I use a combination of a wind meter and reading mirage. I typically gather a minimum and maximum wind reading.
Wind Speed: 3 - 7 MPH
Wind Direction: WSW, 5 o'clock
Gathered Data
Date: 8-21-20
Location:Freedom(32.491555,-112.166558)
Elevation: 1062
BP: 28.4 inhg
Temperature: 88 F°
Powder temperature: 88 F°
Direction of fire: Due E
Distance: 1400 Yards
Wind speed: 3 - 7 Mph
Wind direction: WSW, 5 o'clock
Target size: 5.9 Ft (69") x (17")
Angle of fire: 0
After gathering all the information, I will log it into my D.O.P.E. book. Next, I'll enter the information into my ballistic calculator.
Under these environmental conditions, my ballistic calculator gives me the following:
Drop - 13.6 Mils / 687 in
Spin drift - L .3 Mils / 14 in
Coriolis - L .1 Mils / 4 in
Half value wind - (min 3 mph) .4 Mils / 18 in
Half value wind - (max 7 mph) .8 Mils / 41 in
Danger Space
Next, I can find my danger space and wind error.
The target size is 1.27 Mils tall.
This gives me roughly .6 mils up or down, as room for error (just under 3 feet). That's a large error space. This gives me a lot of confidence that elevation-wise, I can make this shot. At 17 inches wide, or .3 Mils, this leaves little room for error. To be exact, .15 Mils of error, aiming center mass. This means my wind call has to be within 1 mph, or I miss.
Minimum wind is 3 mph at .4 mils, and max wind is 7 mph at .8 mils. This gives me .1 mil adjustment per 1 mph of wind. So, in this situation, I will dial .4 mils for wind and hold center to edge of the target. Because I want a first round hit, my best bet is to hold edge of target. This gives me the most room for error.
Because the wind is coming from 5 o'clock, it will push my bullet left. The bullet will also have less resistance less drop. This will counteract a portion of coriolis and spindrift, as well as elevation. As you will see below, with a 3 mph wind, this will cancel out the spindrift and coriolis adjustment.
Firing Solution
Elevation: 13.5 Mils
Windage: 0 (min) .4 Mils (max)
Always Have A Back Up
I have a D.O.P.E book that all of this information gets logged into. Unfortunately for me, I do forget to log a lot of information. I do, however, also carry a Firing Solutions binder in my pack that has drop tables for every temperature per 10°, as well as all the tables for every correction out to a mile. So, if ever my batteries die or any equipment quits working, I can still get on target.
Using The Back up
Doing math on paper at the range when you want to shoot kind of sucks. It can be slow. If you practice this during dry fire practice, it becomes easier and faster. So, let's walk through it.
This is assuming I have no electronic equipment, aside from a calculator; I will only be using charts and math, no ballistic calculators or range finders. I'll collect data, just in a different order. I'll use the same data as above.
Date: 8-21-20
Location:Freedom(32.491555,-112.166558)
Elevation: 1062
BP: 28.4 inhg (Estimated BP: 28.8)
Temperature: 88 F° (Estimated Temp: 80)
Powder temperature: 88 F° (Est Temp 80)
Direction of fire: Due E
Distance: 1400 Yards (Est 1368 - 1473)
Wind speed: 3 - 7 Mph (Est 3 - 10)
Wind direction: WSW, 5 o'clock
Target size: 5.9 Ft (69") x (17")
Angle of fire: 0
I will know the date, location and average elevation, because this is information I collect prior to getting to my FFP. I have a compass, and can accurately get direction of fire and wind direction.
Distance Formula
The best I can Mil is in increments of about 1/4 mil. (1/2 of 1/2), or maybe 1/8 mil. Not .1 Mil
69" x 27.77 ÷ 1.4 = 1368
69" x 27.77 ÷ 1.3 = 1473
This is where I have to make the best educated guess (I usually split the difference and hope for the best); I know I have at least 1.2 mils room for error. So what gives me the best chance?
Looking at my drop chart per 10 yards
At 1370 I have 13.2 mils of drop.
At 1470 I have 15.0 mils of drop.
I could dial 13.2 mils and aim at the top edge of the target giving me the full size of the target to be off by or split the difference in distance giving me 1420 yards to dial to at 14.1 mils.
(Note: I did not split the difference in mils, but yards. Its an anomaly that 14.1 mils of drop just happens to be the halfway point between the two mil adjustments, and was also half of my distance error. This is not typical because the further a bullet travels, greater is the arc it falls)
At 13.2 mils and the error of 1.2 (size of the target), my total drop before I miss is of 14.4 mils. My drop table tells me 14.4 mils of dope, my bullet impacts at 1430 yards. This means if my target is between 1431, and my max distance estimate of 1470, I miss.
If I split the difference and dial 14.1 mils and aim center mass, this moves my error to .6 mils up and .6 mils down.
That gives me between 13.5 and 14.7 mils.
If my target is between 1390 and 1450 yards, I would still have an impact; however, if the target is at my minimum estimation of 1370 or maximum of 1470, I miss. I would always go for splitting the difference. It just so happens in this example, ether way I would theoretically hit the target.
*TIP* You can measure distance by height and width of your target. It may help to do both.
Barometric Pressure
My rifle is sighted in at 100 yards/ 29.2 inhg / 660 feet of elevation. Estimating that every 1000 feet is 1 inhg gives me an estimated BP of 28.8 inhg. I know at 1000 yards every 1000 feet of elevation I go up, I drop my dope .1 mil. But, because I only Increase my elevation by 400 feet (close to 500), I'll round up. I'll estimate that the difference in barometric pressure will effect it by half or .05. The exception here is that I'm shooting significantly further than 1000 yards. So, just based off of experience, I would estimate dialing down .2 mils. I also have a table for this; on my table at 1400 yards, I would dial .3 mils per 1 inhg. But, I'll split that in half, because I only increase my elevation by 500 feet. So, this gives me .15 mils. I'll round this to .2 mils.
Spin Drift and Coriolis
You will need a ballistic calculator for this, because it will help you determine the time of flight, so this is something you will need to do before you get to the range, or wherever you're going to be shooting. I use a ballistic calculator to develop a table and refer to the table when needed.
The table shows .3 Mils of adjustment between 1300 and 1400 yards for spin drift, and .1 Mil for coriolis.
Eotvos Effect
With a ballistic calculator, this will already be calculated for. With my estimations, this will get lost in the error of using my reticle to Mil the target for distance.
Wind Estimation
I'll face into the wind for about a minute with my eyes closed. I'm trying to feel the wind with face, and listening to the wind in the trees. At the same time, I am controlling my breathing to get into the relaxed and focused mode. If I can barely feel the wind on my face, I'll estimate it's about 3 mph. A definite, but slow, light breeze on my face is about 5 mph. I'll look at the trees and tall grass to make estimates as well. Using this data, combined with reading the mirage, I will estimate a minimum and maximum wind, as I did above. Keep in mind, the target is only 17" wide, so there is little room for error. Unfortunately, wind is very difficult for me to estimate, so I would probably estimate between 3 - 10 mph. I'll reference my wind table of 10 mph full value wind. This gives me 2.2 mils at 1300 yards, and 2.5 at 1400. Because this is not a full value wind, I will multiple this by .75 for 3/4 value. This is also less than 3/4 value, so I will multiple by .75 a second time, giving me a maximum hold of 1.2 mils @1300, and 1.4 mils. @ 1400 I'll split the difference for a max hold of 1.3 mils for maximum wind.
For minimum wind, I'll take 10 and divide by both 2.2 and 2.5, giving me .22 and .25 mils per 1 mph for a full value wind. I don't have a full value wind, so I'll do the same as above and multiply by .75 two times giving me .12 mils per 1 mph wind. At a 3 mph minimum, .12 x 3 = .36 mils.
Minimum wind hold .36 Mils
Maximum wind hold 1.4 Mils
I need to now find how much error I have while still being able to hit the target.
17" x 27.77 ÷ .36 = 1311
17" x 27.77 ÷ .33 = 1430
My room for error is about .03 mils; I can't dial for that. I have very little confidence that I can make this with a first round hit. Ideally, I need to move closer, but this is just an example, so full send it is. My best option is to watch the mirage and estimate based off of that, and then adjust my hold. I will be holding to the edge of the target to give myself the full width of the target. It will be up to luck now, as I have used every skill I have. Focus on the reticle, slow pull on the trigger. I'm watching for impact to make a quick follow up shot if need be before the wind changes. And........?
Firing Solution
Elevation - 14.1 Mils
BP - subtract .2 mils
Wind - subtract .2 Mils
Total Elevation Adjustment 13.7 Mils
Windage
Spindrift - L .3 Mils
Coriolis - L .1 Mil
Wind - min R .4 Mils
Wind - max R 1.4 Mils
Windage Adjustment - 0 Mils
Windage Hold - 0 - 1.4 Mils
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