Introduction

              My name is Trevor Kress and I am currently a senior at Sycamore High School. As a senior, I am required to complete a senior capstone project. For my project, I decided to research something I was very interested in; external ballistics. External ballistics is the study of how the conditions and physical laws of the earth can effect how a projectile travels through the air such as wind, gravity, and drag.

           I primarily chose this topic because I have always had a keen interest in firearms. Ever since I was young I have been around firearms because my family is full of hunters and outdoorsmen. I still remember the day my Dad bought me my first BB gun, and the time when my Mother took me on a hunting trip where I scored my first deer trophy. This hooked me and made me want to know more about guns and how they work. I have also had an equal interest in physics. As a kid I always asked my parents why the moon changed shapes, why the tides changed, and why there was wind. These unanswered questions stabbed at my attention and constantly made me want to know more about the earth and why things are the way they are. When I learned about external ballistics and how it combined two of my interests, I was very excited to research and learn more.

Day 2: Beginning Research

         After beginning my reasearch I have discovered some sources that have contributed vital information towards my topic of interest. One of these sources is the book, Understanding Firearm Ballistics by Robert A. Rinker. This book is useful because it informs the reader about how ballistics is used and also provides pictures and examples to help you understand.

              To help me with my research further, I emailed the National Rifle Association. The NRA will help contribute to my research because they are very knowledgeable about firearms and shooting and someone involved is bound to know something about longrange shooting. Their homepage is www.nra.org. I also have found numerous websites and online sources which have valuable information which I will be able to use for my research. One of these sources is http://www.frfrogspad.com/extbal.htm, this is a website run by a retired Marine who uploads information about rifles and shooting to help readers understand more. Another online source that I have been using is http://www.exteriorballistics.com/. This website is VERY helpful because it provides diagrams which are helpful when trying to understand external ballistics. This site also goes over many things which effect the trajectory of a bullet from varying altitudes, to windage and other factors.

           Overall, I am finding many sources that will aid me with my research and will ultimately help me to understand external ballistics even more.

Zeroing

               Before you do any long range shooting you must make sure that you zero, or sight in your weapon system. This means to match the crosshairs up to where the bullet is impacting a target at a certain range. Most rifles are sighted in at 100 yards. The scopes on these rifles use a "click" system to zero in. The scope has two knobs which turn left and right to move the X-axis of the crosshairs up or down, and the Y-axis let and right. In almost every scope, one "click" moves the crosshairs 1/4 of an inch at 100 yards. The shooter must take an initial shot and D.O.P.E. (Data on previous encounters. in other words, using your previous shots to predict where your next will hit) his scope until the crosshairs and point of impact match up. I used both the link and video below to help illustrate how to sight in a rifle.

http://adventure.howstuffworks.com/outdoor-activities/hunting/traditional-methods/rifle-scope4.htm

Mil Dots

                  While watching a show about snipers I learned that shooters use mil dots to help them fire at long distances. I did further research with videos and various websites and found out that mil dots can be used effectively for targets at long ranges. Once your scope and weapon system are in sync, it is important to know and understand the specific the mil-dot system of your scope. A mil, or milliradian, dot is a small dot that is placed on both the X and Y-axis of the crosshairs. These dots allow the shooter to estimate certain ranges by offering a known length at a certain distance. Ten of these dots are placed on both axis and can be used very efficiently in a grid-like fashion if you know what you are doing. For example, the length between two mil dots (one mil) at 100 yards is about 3.6 inches. Therefore 10 dots, or the total length of either the X or Y-axis is 36 inches or 3 feet. So if you are looking at a target through your scope that you already know is 18 inches across, and it measures 5 mils, you know that that specific target is 100 yards away. If the same target measured only 2.5 mils across, it would be 200 yards away and so on. Mil dots can be used very well if you know what you are doing, but this takes much practice. The video and link below help to show how mil dots are used.

http://www.boomershoot.org/general/TruthMilDots.htm

            

Correcting for Gravity (Bullet Drop)

          I have learned that one of the most common factors that affect a bullet while in flight is gravity. Gravity is the force that pulls every object down to earth at approximately 9.8 meters per second, or 32 feet per second. Gun manufacturers have realized that bullets start dropping the second that they leave the barrel of the rifle, but most people do not know that the gun makers have developed a way to make gravity less effective on a bullet after it is fired. To do this, they added a slight upward slope to the barrel. This causes the bullet to actually rise above the original line of trajectory of the projectile. It may seem like a bullet is traveling in a straight line, but it is actually traveling in a very oblong "parabola" or curved path. Therefore, after the bullet is fired it raises above the shooters line of sight, then drops below the line of sight. When you are making a zero, the point of impact is where the line of trajectory and shooters line of sight cross paths. The picture below will help explain.

              To help you guess exactly how much a bullet will drop at a certain distance, a number of things must be known. First is the muzzle velocity of the specific weapon system you are using. For my example I will be using a standard 30-.06 hunting rifle which has a muzzle velocity of ~2,800 FPS. Another thing is the shape of the bullet. I will be using a streamline (or pointy) bullet rather than a blunt bullet. This is because the blunt bullet will be in the air longer, thus allowing gravity to have a greater effect. At 100 yards, your zeroed-in rifle should not drop at all. (Assuming you sighted your rifle in correctly) Lets assume we are shooting at 500 yards, this is equal to 1,500 feet, therefore the bullet will be in the air for approximately half of a second. In this half of a second, gravity should pull the bullet downwards around 16 inches. If we go back to our mil-dots we find out that at 500 yards, one mil is approximately 18 inches. Therefore to compensate for this drop, we would have to aim our crosshairs ~1 mil dot above the target before firing.
          

http://www.angelfire.com/tx5/snipersustainment/External.html

Drag

         Drag is another factor which affects a projectile while in flight. Drag causes a loss of bullet speed as the bullet pushes its way through the atmosphere. Drag also allows gravity to have a greater effect because the bullet is in the air for a longer period of time. Other things that may influence drag is the ambient air density, or the measure of humidity, air pressure, and temperature.

             As temperature increases, there are less air particles for the projectile to impact while in flight. Therefore, the hotter it is, the further your shot will travel. Also, as you increase in altitude, the thinner the air becomes, causing the same effect and ultimately causing a further flight period.

             Humidity is the measure of how much water vapor is in our airs atmosphere in a given area. Most people think that wet, sticky air will cause the bullet to travel less than dry air, but they are wrong. Water vapor has a density of .8 grams per liter while dry air has a density of 1.225 grams per liter. The less dense the air is, the less drag there will be. Therefore the more humid it is outside, the further the bullet will travel.

Wind Deflection

            Wind deflection is another factor which has a great effect on a bullet. During wind deflection, gusts of air push the projectile and alter it from its original line of trajectory. A sudden gust of wind can change 50% in a matter of seconds and this causes the need for an adjustment. A shooter could deal with a 2-3 MPH winds and not feel a thing, or multiple crosswinds that are each traveling in different directions.

           A good technique to compensate for wind deflection is to first find out which direction the wind is blowing. This allows the shooter to determine how much "value" the current wind has. There is a simple clock-like diagram that you can use to find out the value. If you look at the diagram below you can see this diagram. Winds blowing from 2, 3, 4, 8, 9, and 10 o'clock are almost perpendicular to the shooter and will have the most effect on the bullet. These values need the largest adjustment. Winds coming from 1, 5, 7, and 11 o'clock are not fully perpendicular to the shooter so they are considered half-value, but still require an adjustment. Other winds are almost straight on and do not require adjustment. A lot of shooters then use vegetation around them or hand held wind devices to figure out how quickly that wind is moving.

         

           A formula for wind deflection is  D=W (T1-T2), where D is the deflection, W is the cross wind velocity in Feet per second, T1 is the time of flight for that range, and T2 is the range divided by the muzzle velocity.

          Imagine a 30-.06 round at 500 yards, or 1,500 feet. A 10-MPH wind moves at 14.76 fps and the muzzle velocity is ~2,800-3,000 fps, making the time of flight .744 seconds. If you run the formula, you find that the correction is 5.84 feet.



Other Factors

            Throughout my research i discovered many variables that impact a bullet while in flight, some making huge differences, and others small. Wind, gravity, and drag all have a fairly significant effect on the projectile, but they are not the only ones shooters must keep in mind. Another important factor to consider is gyroscopic drift.

            All rifles have a thing called rifling, or small grooves in the barrel which put a spin on the projectile. This spin on the bullet stabilizes it in flight, much like how a quarterback puts spin on a football. This constant rotation causes the bullet to pull to one side. I discovered that 30-.06 bullet at 1000 yards will drift ~13 inches due to the rifling effect. So if we are shooting at 500 yards, we would have to use our mil-dot system to compensate for the 6.5 inch drift on the bullet. It may not seem like much but it could make a huge difference in combination with a strong wind and other determining factors.



            Another small factor is the coriolis effect. A french professor discovered the natural law which states that moving things such as projectiles and water move in relation to the natural spin of the earth. They move to the right in the northern hemisphere, and to the left in the southern hemisphere. What this means is that the natural spin of the earth could actually throw the bullet off of its original line of trajectory. I did some research and discovered that a projectile deviates about 6 inches at 1000 yards from the earths rotation in the northern hemisphere. However, maximum drift occurs when the bullet is fired towards the southeast and minimum drift when fired to the northwest. Generally a bullets curve will be about 1/10 of an inch at 125 yards in the central US, so there would not be a huge difference at distances around 500 yards.

http://www.appliedballisticsllc.com/index_files/SpinandCoriolisDrift.htm

Conclusion

            At the beginning of my research I did not know that much about external ballistics, but over the past few weeks I have learned a lot. I had no idea that there were so many variables which affect a projectile while it is in flight. I also did not realize how sophisticated long-ranged shooting was. I learned that shooters are expert mathematicians who can quickly and efficiently use their knowledge about the Earth to accomplish their goals. Long-ranged shooting is an art that must be perfected if it is going to be brought into real situations or conflicts. If it is not perfected, the shooters could lose more than just their shot, they could also lose a life.

Extras

        These are a few videos of some of the greatest shots ever taken. They go into detail and show how truly amazing long-ranged shooting is and how many things snipers must take into consideration before pulling the trigger.

Longest Ever Kill Shot

Mile-Shot

Nice shot