External ballistic effects change the bullet trajectory in so many ways that a naked eye can’t see. The obvious effects consist of gravity and wind. There are other 28 weird things affecting the bullet in flight. There are physical and environmental elements affecting the bullet.
Let’s reveal the 28 weird things that make a bullet fly the way it does with visual illustrations.
What is External Ballistics?
External ballistics associates with firearms. It deals with the behavior of the bullet in flight after leaving the rifle barrel. The main forces acting on the bullet include gravity, wind, and aerodynamic drag. Check out this in-depth resource from Hornady here.
These subtle effects are generally irrelevant for close-range shooting. The reason is close range shots can hit the target without significant precision.
Minor effects become important for extreme long distance shooting. Every calculation must be accounted for.
How To Shoot Long Range Better By Understanding Bullet Ballistics
A bullet in flight is affected by gravity, wind and aerodynamic drag. These are the three main things everyone understands, even for an inexperienced shooter. But, for precision shooting, there are way more things affecting how a bullet travels.
Basic Bullet Trajectory Physics
The bullet comes out of the barrel at a certain muzzle velocity toward the target. Gravity and aerodynamic drag start to slow the bullet down and fall. A bullet with high muzzle velocity goes further before it starts to fall.
The shooter compensates the bullet drop with holdover calculation.
1. How Does Bullet Trajectory By Caliber Affect Performance?
Bullets come in different shapes, weights, and sizes. All contribute to how a bullet performs in flight. Every caliber bullet needs optimal mass and shape to maximize velocity and energy.
There is a large selection of precision ammo recipes to make the perfect bullet for a specific distance, target, and barrel length.
The bullet caliber affects flight trajectory so much, it’s like rocket science.
Reloaders in the precision communities use tons of data to make customized rounds. They use a lot of data references, testing, and charts to perfect their bullets to counter those external ballistic effects.
The profile of a bullet or the shape has an impact on bullet spin and gyroscopic stability. The more aerodynamic the bullet profile is, the better the bullet can fly.
That means it will perform well without losing velocity, stability, or tumbling.
A low drag bullet profile with boat tail increases long-range kinetic efficiency and speed. Bullets with a low drag coefficient decelerate less rapidly, and that decreases the lateral drift caused by crosswind.
Some people go a couple of steps further to experiment with various powder charges.
2. What Causes Bullet Yaw
Yaw is the motion when the bullet is turning sideways or tumbling in flight. A bullet of a given mass is flying through the air is experiencing drag — the greater the bullet velocity, the greater the retardation.
This is one of the most common external ballistic effects any flying object has.
Aerodynamic drag will slow down the bullet because of the spin. The faster the bullet can spin, the less likely it will yaw.
So how can you control the spin? Well, this goes back to what type of gun barrel the bullet is shooting out of and how fast the bullet leaves the barrel.
The barrel twist rate influences bullet spin and stability in flight.
3. Does A Hot Bullet Affect Performance?
The temperature of the bullet definitely affects external ballistic effects. Yet, I don’t think there is an accurate way to measure it. I have seen people measure how hot the barrel gets, but not measuring the bullet specifically.
When a round is fired, the pressure inside the chamber fills up very fast, and then it bleeds off as soon as the bullet has left the barrel.
The bullet has a slightly larger diameter than the barrel. It’s tight, so it creates a good seal for the pressure to build up when the gunpowder ignites. The pressure shoots the bullet out of the barrel.
4. Why Does A Bullet Have To Spin
The bullet spin keeps the bullet stable in flight. It makes sit fly straight and fast as it spins to maintain exactly in line with its flight path. Rifling from the barrel spin up the bullet as it shoots.
This is the same concept for throwing a football.
The spin makes it fly straight.
Bullet precession is also called gyroscopic precession. The bullet has a wobbling motion once it leaves the barrel, which causes instability.
Bullet Weight distribution is also a factor. It’s the same reason why car wheels need to be balanced with lead weights. When a car is traveling at a certain speed, they wobble due to precession and the car vibrates.
The same happens to the bullet in flight.
Bullet spin creates angular momentum. This creates this interesting external ballistics behavior called precession. The bullet will spin in flight, but it will also cartwheel around its axis as it flies.
A bullet tip is not always concentric, which means it’s not perfectly round. When the bullet is experiencing precession, it swerves due to nutation. It creates small circular movements at the bullet tip.
The amount of bullet instability is due to the bullet length and the spin rate. The longer the bullet, the faster it spins and has a harder time getting stabilized.
This is why the profile of the bullet and the angle of attack are very important for stability.
7. Bullet Angle of Attack
The length of the bullet is also a factor. Some low drag bullets are optimized for long-range ballistic performance. The bullet length is also a critical factor for performance.
The length between the center of pressure and the center of gravity controls the angle of attack.
This allows the bullet to adopt an angle of attack between the bullet’s spin axis and the flight path. This is also called the Poisson effect (the bullet’s nose is titled above the line of the trajectory).
It’s just like how a football looks in flight.
Aerodynamic forces will push on the center of pressure over the center of gravity — this causes the bullet to tumble. By spinning the bullet faster enough, the gyroscopic effect makes the tumbling stops.
This is like how a rocket flies into space.
8. Spin Drift
When a bullet spins, naturally it will drift to the left or to the right. We call it spin drift. It’s determined the direction of the barrel rifling.
A clockwise spin will drift to the right and a counterclockwise spin will drift to the left. Most rifle barrels have a right twist.
Spin drift is a deflection generated by the gyroscopic motion of the bullet in flight. The deflection direction is toward the direction of the spin.
The deflection amount is dependent on the bullet length, flight time, and spin rate.
The bullet wobbling is lessened over the distance. When it has stabilized, the bullet’s longitudinal axis no longer points in the direction it’s flying. This is Yaw of Repose.
9. Barrel Twist Rate Affects Bullet Differently
The twist rate of the barrel has a direct impact on this. Most barrel rifling has a right twist. The twist rate refers to the rate of spin.
For example, A barrel with a 1/10 twist rate means 1 revolution of a bullet spin in 10 inches.
Heavier bullets need a tighter twist rate because it can stabilize in flight. The rate must be constant for the best accuracy. Different barrel length and caliber need the optimal twist rate.
The same bullet fired in two different twist rate barrels can have different velocity, but it’s insignificant.
10. Gain Twist – Variable Twist Rate
The rifling twist rate that gets tighter over the length of the barrel is called a gain twist. A barrel might start with 1/12 twist and finish with 1/8 twist.
There is some evidence that it can increase velocity with certain caliber bullets but is hard to prove.
11. Magnus Effect
A spinning projectile induces pressure differences due to deflection by twisting its axis of symmetry of its flight axis. The air flowing around the bullet in flight creates a high-pressure zone, which pushes on the bullet’s center of pressure.
The Magnus force will always be perpendicular to the sideways vector of the wind acting on the bullet. This effect also acts upon the bullet’s center of pressure. This causes the bullet to yaw.
The center of gravity of the bullet is what follows the line of trajectory. When the center of the pressure is different, it will torque the bullet to the side.
12. Yaw of Repose
Magnus effect induces yaw, then it changes the aerodynamic stability. It also changes the ballistic coefficient and direction of travel. Also, it causes most of the spin drift.
A stabilized bullet creates a yaw angle. It's due to aerodynamic drag force acting on both the center of pressure and the center of mass.
The yaw angle equals the yaw of repose.
The direction of the bullet spin determines the yaw of repose.
The longer a bullet travels through the air, the more time the environment is affecting it. These include gyroscopic drift, wind, gravity, air density, temperature, and kinetic energy.
The longer the distance to target, the bullet will start to diverge. So precise holdover and caliber selection are important for long-range shots. Small imperfection at closer range magnifies at extended ranges.
Top External Ballistic Effects – Environmental
14. Crosswind & Aerodynamic Jump
The crosswind is directed either from the left to the right, or the right to the left horizontally from the direction of the bullet’s flight path. A left crosswind will drift the bullet more to the right, and vice versa.
The longer the wind pushes on the bullet over the distance, the farther it drifts. This changes the bullet point of impact (POI).
Aerodynamic jump describes the bullet's change in vertical impact when encountering a crosswind. It's a term less commonly used in applied long-range ballistics. The jump in the vertical deflection of a crosswind.
The wind from the RIGHT will cause the bullet to hit high and vice versa.
The cheapest solution is to use a Dope Disc Windage Solver since most ballistic programs don't account for it.
Tailwind speeds up the bullet by boosting it, and it decreases the drag. A headwind will create more drag on the bullet and causing it to drop.
16. Vertical Wind
Nobody ever talks about this one, but it’s interesting. A downward vertical wind will cause a bullet to drop and vice versa. This is common in mountainous areas where vertical winds are caused by terrain.
A bullet can also experience all three types of wind. Each component of the wind will influence the bullet.
All these external ballistic effects relate to one another as a system. The more you understand what each element is doing, the better your precision. You can pair the best rifle with the best bullets for long precision shots.
17. Do Bullets Actually Rise?
The bullet does rise a little bit, but it will never rise above the axis of the rifle barrel. The bullet leaves the barrel with little or no physical support.
The bullet trajectory physics has a parabolic arc if you pay attention as the bullet flies downrange using a spotting scope. You will get the illusion that the bullet is rising a little bit and then fall.
The line of sight through the scope is slightly above the bore is one reason. Also, the barrel for some rifles is slantedslightly upward to compensate bullet’s initial drop.
18. Bullet Kinetic Energy
The gunpowder explosion creates an enormous amount of pressure to shoot the bullet out of the rifle barrel. The energy of the bullet is commonly measured in foot-pounds.
The muzzle energy = muzzle velocity ^2 X bullet weight (grain) / 450437
Why does kinetic energy affect a bullet’s performance?
Kinetic energy determines the terminal ballistic performance. In other words, how much stopping power it has to knock down a target.
Kinetic energy goes from as little as 135 ft-lb from a 22LR to 5000 ft-lb 338 Lapua Magnum.
19. Coriolis Effect
The un-noticeable spinning motion of the Earth creates the Coriolis effect by bullet deflection. It’s negligible at medium distances, but beyond 1000 yards or more, it has more influence to keep your off target.
A flying object in the Northern hemisphere tends to deflect to the right.
The horizontal component of the bullet trajectory is depended on the latitude.
The deflection is not east or west but to the left or right relative to the shooting direction.
The effect is the greatest horizontal effect is at the pole, and zero at the equator of the earth.
The vertical component of the Coriolis effect is based on the direction you’re shooting on the Earth. The effect is neglected when shooting north or south.
When shooting east, the point of impact will be high. When shooting west, the point of impact will be low.
20. High Angle Precision Shooting
Long range bullet trajectory has a greater external ballistic effects impact at greater than 45 degrees shot angle. Understanding how much to compensate for bullet drop is very important.
So do you aim higher or lower when shooting downhill? When shooting uphill, the shooter must hold the sight below the target.
The shooter must aim below the target when shooting downhill to compensate for shooting angle. Because the bullet drop won’t be significant relative to the point of aim.
So how does shooting uphill affect trajectory for a bullet? It's the same for shooting downhill.
This is called high angle shooting.
Professional shooters use shooting angle calculator to find the angle modified range. They also use range finders and cosine angle indicator to be more precise.
Temperature controls the air density which can change air density. This determines the drag on a bullet. Cold temperatures cause air to be thicker which creates more drag.
Warmer temperature causes the air to be thinner which reduces drag on a bullet. It increases a bullet’s performance to travel farther than in colder weather.
As a rule of thumb, roughly a change of 20 degrees results in about 1-minute difference in point of impact, so keep that in mind when you calculate MOA or any high angle shooting.
A good reference will provide you with various calibers performance data.
22. The Effects of Humidity on Bullet Trajectory
Humidity has some impact on external ballistic accuracy. It reduces a bullet’s spin performance. Air containing a high amount of moisture is denser than dry air because a water molecule weighs less than a dry air molecule. Cold temperatures can definitely affect this.
Higher humidity results in thinner air. It makes the bullet to travel easier through the humid air. As air density increases, the bullet experiences more resistance, which causes the bullet to slow down. However, the most influential elements that affect accuracy is the density altitude.
23. Density Altitude
Bullet performs differently at various altitudes. Higher altitude results in thinner air, and the bullet flies faster. The amount of bullet drop on the way to the target is all about how fast the bullet travels.
The faster it flies, the less it drops.
Density altitude is an environmental variable that influences the speed of the bullet. But, wind and gravity have bigger influences on bullet trajectory than anything else.
24. Rifle Barrel Temperature
The higher the barrel temperature, the more it decreases bullet accuracy.
Cold bore shot is a term used in the precision shooting community to test the accuracy of a bullet from a cold barrel to a hot barrel.
25. Bullet Weight Matters
The weight of the bullet affects accuracy and terminal ballistics. It’s an important factor in the military, law enforcement, and hunting applications. The mass of the bullet combined with enough muzzle velocity delivers significant energy necessary to knock down a target.
The center of gravity of the bullet is the reference point that follows the ballistic trajectory. The flight performance of the bullet is depended on the optimal distance between the center of pressure and the center of gravity.
26. Bullet Profile & Ballistic Coefficient
Ballistic coefficient is the measure of the bullet’s ability to overcome aerodynamic resistance in another word, how streamlined a bullet’s shape is. Each bullet has a different profile and shape that yield a unique ballistic coefficient.
Ballistic Coefficient (B.C) = Drag of the standard projectile/drag of test projectile. This ratio is essentially a measurement of air drag. In layman’s term, the more streamlined bullets can maintain velocity better. Slightly heavier bullets with the same shape can outperform lighter bullets at the same muzzle velocity.
27. Boat Tail Bullet Means Low Drag
Boat tail bullet combined with the optimal caliber and bullet mass is more stabilized in flight, which yield better accuracy. The bullet has its tail tapered, which has the name “boat tail” because it resembles the stern of a boat.
Boat tail bullet has a higher ballistic coefficient. It helps to overcome air resistance and wind deflection. Most importantly, it reduces drag to improve the spin performance of the bullet in flight.
28. Ballistic Tip Affect Accuracy?
In the late 19th and 20th century, the military wanted a more aerodynamic bullet to deliver a higher degree of accuracy, kinetic energy, and range. This lead to the Spitzer bullet design. The pointy bullet tip is also known as the spire point bullet.
You have probably seen full metal jacket bullet tip and ballistic tip. Any difference in them?
Full metal jacket is ideal for long range shooting because of its streamlined shape. The ballistic assembles the same bullet profile as a full metal jacket, but it’s designed for expansion upon impact.
Hopefully, you’re not brain fried after reading all these. I tried to keep this as simple as I can without going into too much detail. If you want to know more about external ballistics, then I highly recommend the following:
Best External Ballistics Reference Books To Buy
Applied Ballistics For Long Range Shooting 3rd Edition
This book covers very matured modern science of external ballistic effects. The author, champion shooter Bryan Litz breaks down the science of accuracy into layman’s terms that are easy to understand.