“Looking down range, you see the wind drifting steadily from left to right. It appears to be holding at a constant 10 MPH, for now. You ranged your target at about 620 yards. As you wipe the sweat from your brow, you look down at your portable weather unit. It reads 91 degrees Fahrenheit and 80% humidity at an altitude of 1340ft above sea level.
You double check your DOPE book for your .308 Winchester bolt gun. Doing some quick calculations, you lay back down looking through you scope. The wind is still holding steady at about 10 MPH from left to right. Right before you squeeze the trigger, you dial up 4.4 mils and hold about 1.5 mils left. You squeeze the trigger slowly until finally the loud bark from the rifle sends the 168gr BTHP projectile down range with a muzzle velocity of approximately 2700 fps. As the bullet is screaming through the air, you quickly re-center just in time to catch the trace. You see an impact towards the right edge of the man sized steel silhouette. You think to yourself, I little right, but a hit is still a hit. You adjust and settle in on your next target and start the process over as the clock continues to count down…”
Having discussed a lot of the external ballistic factors in the previous article and what exactly they mean and how they affect your shot, hopefully the above statement (which is an excerpt from the introduction article I wrote) is starting to make a little bit more sense, at least in terms of why all of that data is important and what it is going to do to my bullet in flight.
With that out of the way, lets begin into some of the other ballistics terms that you should be familiar with should you decide to reach out a bit further and into the extreme long-range realm. As I stated in external ballistics 1, for most of you, the terms covered in this section aren’t really anything you have to concern yourselves with while shooting long range as the effect they would induce would be almost negligible. With that being said, regardless whether you plan to shoot out to distances where you would need to really understand this information or not, it doesn’t hurt to have at least a basic understanding because you never know where your long-range shooting endeavor will go.
To begin, we are going to cover the terms we are going to be discussing in this article. Like the previous article, we will go over the definitions before getting into examples which will help us better understand how these apply to our shooting. These definitions will be put in my own words so it should hopefully be easier to comprehend.
- 1)Parabolic Arc: for our purposes, it is the type of curve that represents the flight of the bullet.
- 2)Bullet Yaw: when the nose of the bullet begins to point in a direction other than the direction of its forward travel. This can happen at shorter distances but generally is not a factor until shooting out to further distances, or in severe cases the bullet begins to tumble due to instability in flight. This will be discussed later in the series.
- 3)Magnus Effect: an effect that causes the bullet to yaw. This can be vertical and/or horizontal. The magnus effect can contribute to Aerodynamic Jump and spin drift.
- 4)Aerodynamic Jump (A.J.): deflection induced on the bullet affecting its vertical position the parabolic arc. Simple, when the bullet climbs or descends in a manner that is unnatural to the bullets flight path.
- 5)Spin Drift: the horizontal deflection of the bullet in the direction of the bullets rotation that is nor contributed to wind..
- 6)Coriolis Effect: effect on a bullet where the earth’s rotation must be taken into account.
- 7)Transonic Zone: the area where the bullet transitions from supersonic to subsonic due to loss of speed over distance that momentarily disrupts the bullets flight
Starting from the top, let’s begin with parabolic arc. I realize I could have discussed this in the previous article as it does apply before reaching the extreme long-range distances, but for the purposes of understanding these terms, I decided to include it in this article. Anyway, as described in the definition, it is the shape of the bullets flight path. Instead of a gradual climb to the apex of the curve and then a gradual descent creating what I like to call a rainbow type arc, it has a lower gradual climb to the apex before descending in a more rapid manner the further the distance gets. To relate this, imagine you throw a football at someone who is 10 yards away. I understand for some of you this is a feat unto itself, but go with me on this journey.
You give the ball everything you’ve got and throw it as hard and fast as you can, and you manage to throw a perfectspiral. At that distance the ball travels in almost a straight line. As the distance increases you have to angle slightly upwards and you begin to see more of an arc in the balls flight path. Out to 20 yards it seems like a gradual rise and fall. Out to 30 yards you throw the ball and because of the angle you threw it, it begins to rise and then start dropping a little more rapid. This is because your gravitational pull remained the same but the forward momentum begins decreasing. This is where you really begin to notice the parabolic arc. Now you throw a Hail Mary out to 70 yards and the ball begins its’ flight at a very sharp angle upward before reaching its’ apex. At this point the ball appears to start dropping like a rock almost straight down because the ball has lost so much forward momentum that the ball continues to drop at the same rate while barely moving forward with what remaining speed it has left. Your bullet does the same thing in flight.
Next up is bullet yaw. Like the definition described, it is when the nose begins to point in a direction other than the bullets path of travel. There are many factors that can contribute to a bullet yawing, not all of which are external.
First is the magnus effect that we described earlier. Without getting too scientifical, because of the rotation of the bullet and how air moves around it, it begins to nose up, down, left, or right. Due to the amount of friction put onto the bullet by the air molecules, it does not matter if wind is present or not, but wind will generally increase this based on the direction the wind is coming from and its strength. Imagine you are driving in a car and there is absolutely no wind. Now you roll your window down and stick your hand out. Even though there is no wind you can still feel the wind blowing past your hand. This is because of the speed at which you are moving basically making your own wind, hence how airplanes fly. This is the friction I am talking about. This can also be exaggerated due to bullet design.
Bullet design is something we will get into more on the next article. Some bullets by design are just more susceptible to certain external factors. Last would be the bullet came out of the barrel already pitched. This is also something that will be discussed later on in the series.
Next in the lineup is Aerodynamic jump (A.J.). To be honest this sounds way more in-depth to understand than it really is. Per the definition, it is deflection induced on the bullet affecting its vertical position the parabolic arc. Basically, as the bullet is travelling along, if the bullet makes a minor jump up or down while travelling (from my experience, usually up) then that’s what this is referring to. For example, have you ever thrown a frisbee and as it is traveling it suddenly jumps up or drops a little bit but still continues straight… that’s pretty much it… at least visually. I am not an aerospace engineer so I am not sure what exactly causes a frisbee to do that but I imagine that’s what it is and it makes for a great example to understand what A.J. is.
Next in the lineup is spindrift which is the horizontal deflection of the bullet in the direction of the bullets rotationthat is not contributed to wind. This is another one that is pretty easy to understand. To get the visual, imagine a side view of a tire to represent the base of the bullet. Now, imagine it is rotation and just barely making contact with the road. Think of the road as the air molecules inducing friction onto the bullet. As it moves forward it is rotating. The whole time it is moving forward and rotating, the air molecules are causing friction onto the bullet making it want to pull into the direction of its rotation. Another visual would be to think of a golf ball. If you are anything like me when you golf you tend to slice the hell out of the ball. As the ball is moving forward it is veering of course in the direction of its spin. To be clear, that is an over exaggeration as the bullet does not go that far off course, but it gets the point across. Side note, the further out you shoot, the greater the shift. Again, it is no where near as drastic as the golf ball, but it still can mean the difference between a hit and a miss.
We are almost done, I promise… Coriolis Effect is the effect on a bullet where the earth’s rotation must be taken into account. Basically, depending on what caliber and how far out you are shooting, your time of flight over that distance, your location on earth, and in which direction you are shooting (north, south, east, or west) you have to take into account the earth’s rotation underneath the bullet while it is in flight. The earth rotates at about 1,000 MPH, but this really only comes into play if you are shooting out past 1,000 yards (again, this depends on what cartridge you are shooting) and even then, it’s a very small adjustment. Moral of the story… it is really important to track and record your data when you shoot precision rifle.
Last up is Transonic Zone which is the area where the bullet transitions from supersonic to subsonic due to loss of speed over distance that momentarily disrupts the bullets flight. In simple terms, as the bullet slows and begins to transition to a forward speed of less than the sound barrier (approx. 1129 fps at sea level), it breaks the sound barrier which is like hitting a super thin wall which causes it to destabilize in flight. Depending on the bullet design, it can either regain stability after passing through or begin to tumble and drop like a rock. For those military folks out there who have heard of the term max effective range, this is where that comes from. Now, just because this is looked at as a bullets maximum effective range does not mean it cannot be pushed further. Not all cartridges and bullet designs can be pushed further, but keep transonic zone in mind as this will come up in our next article as it will come into play.
To wrap this up, external ballistics 1 and 2 should give you a much better insight into the realm of the ballistic factors your bullet is going to have to fight. As always, I do not claim to know everything and there is a whole lot more out there when it comes to ballistics, this just goes a little below the surface to help you understand what your bullet is up against while in flight. I strongly encourage all my readers to not just take my word for it. Do some research on your own as there is a lot of great information out there that goes way more in depth and nerdy than I have time for. It’s a fascinating subject if you’re into it.