Precision Shooting 1-Part 2: Optics, Data and Logic
In this first scenario I mentioned that if you have your range/distance to target, a solid rifle, a solid zero and a good quality scope, a data card showing you your holds or minute of angle adjustments, the animal is within 700 yards and the wind is right, you will take the shot and hit with confidence. This is true, but perhaps you are thinking how to obtain all of this? Well, let us say that you already have a solid rifle such as the likes of an Ed Brown or a GA Precision Rifle with a Leopold Mark 4 or Night Force scope on it and you have it zeroed for 100 yards. Your next step would be to obtain your muzzle velocity so that you can order or make up a data card. The instrument that you would use to obtain your muzzle velocity is called a “chronograph”.
The chronograph is a simple instrument to use. One of the chronographs that I own is manufactured by a company called “Chrony”. When completely opened up and ready to use, it measures approximately 4 inches by 10 inches. You set this up on a bipod approximately ten feet in front of your shooting bench and shoot over the top of it. When you do this, it will give you a velocity reading. I suggest using the best average velocity of five rounds fired. Be careful to note the temperature and barometric pressure or elevation that you were shooting at because you will need this data to make up your data card.
Once you have obtained your bullet velocity, you can contact a company by the name of “Ballistic Cards” located in Atascadero, California. You simply give them your velocity information along with the temperature, barometric pressure/elevation and humidity (if possible) along with the type of bullet that you used and they will make up a custom data card for you.
This “data card” will display what adjustments you will need to make on your scope or where to hold on your reticle (if using a mil dot or minute of angle reticle) for your distance to targets in either yards or meters. The card is set up so that the yardage is depicted from 100 yards to 1000 yards in 50 yards increments. So the next time you are hunting, and you range your target you will know almost exactly where to hold or what adjustments to make. Now granted the card that you had made up was near perfect for the elevation and temperature that you shot at, but won’t be if you do go into a cooler and higher/lower elevation.
Temperature and elevation both affect the bullets point of impact. The colder the weather the denser the air, which in turn creates more drag on the bullet. The warmer the air the less dense it will be and the drag will be less. Elevation has a similar effect on the bullet. When you are at a higher elevation the air is thinner, and always cooler. The temperature will decrease 2 degrees Fahrenheit for every 1000 feet in elevation gained. So at 4000 feet elevation, if the outside ambient air temperature is measuring 70 degrees, it will decrease to 58 degrees when you climb up to an elevation of 10,000 feet. This temperature change is hardly enough to have an affect on the bullet, but it does affect it. Because the air is thinner there will also be less drag, which means that the bullet will shoot flatter or theoretically have a flatter trajectory causing the bullet’s point of impact to be higher.
Having current meteorological data can aid the hunter and precision shooter if they utilize the various different software packages available such as, “Exbal”, which will run on a Pocket PC as well as your home computer. “Sierra’s Infinity” software is another good software package that also runs on your home PC.
This program is set up so that on the screen of your Pocket PC there are three columns or data entry points. The first column consists of:
1) The bullet’s ballistic coefficient, which can be obtained by calling the manufacture or looking in your Exbal reference library.
2) The Chronographed velocity
3) The zero distance of your rifle
4) Site height from the center of bore to center of scope.
The next column consists of:
2) Barometric Pressure
And the third column consists of:
1) Wind Speed
2) Wind direction
3) Angle or Cosine of the Angle that you are holding on.
4) Target Speed
When this is inputted into the Pocket PC’s Software, it will give you your holds/adjustments based on the actual field conditions including a complete data card.
Now, when I have spoken to people regarding this, they have laughed at me stating, “I don’t have enough time to do all of this”. My humble response is, “well, of coarse you don’t. But you do have time to do it before you start walking”. At this point the person that I am conversing with usually gets stuck in-between their egos, thirty years of hunting experience and trying to comprehend what I just said.
This whole approach is basically, logical planning. After the data has been imputed into the Pocket PC, it will display a data card. All you have to do is copy that information onto some write in the rain paper with a black Sharpie marker and put it in your pocket or tape it to the side of your rifle. Now, for the most part, the next time you range your quarry, and calculate the corrected for gravity distance to target, (cosine X mil or moa hold = corrected for gravity hold to adjust for your target), all you have to do is look at your data card and make your scope adjustments. However, you can also make yourself a “Range Card”.
Making a range card
When we were at our over-watch site we were glassing for animals, indicators and target reference points. For all intensive purposes, let’s say that we found 5 target reference points (TRP’s). They are a small stream where we see water filled tracks heading away from us, an area of mulled up dirt on the bank of a watering hole, several intersecting animal trails, flattened grass and droppings.
At this point we will utilize what is called a “range card”. A range card as depicted in the following sketch is a layout of the area and the location of the TRP’s that you sketch. This range card contains no information other than the categories we will be utilizing and is very similar to the one that military precision shooters use.
Click here for larger image.
We either look through our laser range finder to locate the distance to target or utilize the mil-dot or minute of angle reticle in our scope. The formula for the mil dot reticle is: [size of target in yards multiplied by 1000, and then divided by the number of mil-dots the target is sitting in, or size of target in inches multiplied by 27.77 and then divided by the number of mil-dots the target is sitting in]. For a minute of angle reticle, the equation is: [size of target in inches multiplied by 100 and divided by the number of minute of angle the target is sitting in]. But, for all intensive purposes, let’s use a laser range finder. We simply look through the laser range finder, push the button and the range is displayed for us.
So, for the five targets: 1) the water-filled tracks is 482 yards, 2) The mulled up dirt on the bank of the watering hole is 611 yards, 3) the intersecting animal trails on the hillside is 711 yards, the flattened grass is 888 yards and the droppings are 216 yards. Figure three shows an example of a completed range card.
However, this acquired distance to target is the uncorrected for gravity distance. As in this scenario we are sitting on a hillside looking down at the TRP’s, we need to correct for this in order for our bullet to hit where we have aimed. If we do not correct for gravity, the point of impact will be higher than intended. Let me explain why and then how we correct for this.
Correcting for the effects of gravity – “Angle Shooting”
There is a physical ballistic problem encountered when shooting on angles that causes the bullets point of impact to hit high.
Shooting on angles is what every hunter experiences while hunting in mountainous terrain. Sheep hunters and deer hunters alike know all to well, that if their target is up or down on an angle, that they must aim low because the bullet will impact high. The reason for this has to do with gravity and the adjusted site height above the bore of the barrel.
When we zero in our rifle at 100 yards, we are shooting on a flat plane with the full force of gravity pushing down on the bullet. In order to zero properly and get the bullet to strike the bulls-eye, we need to adjust the sight height above the bore of the barrel for this particular condition, (shooting on a flat plane) so that when the bullet leaves the bore of the barrel it arcs up into the full force of gravity, and then drops down onto the bulls-eye.
However, when we shoot on an incline or decline (up or down on an angle) the force and effect of gravity is less on the bullet; but the sight height above the bore of the barrel remains the same, or adjusted for shooting on a flat plane.
Because of this, the bullet will have a flatter trajectory and strike the target higher than where our intended point of aim was. It is imperative that when we are shooting up or down on an angle that we eliminate the guesswork, and correct the straight line distance to the target or “sloped distance,” to the, corrected for gravity, distance to target.
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