External ballistic effects exert numerous subtle influences on a bullet's trajectory, often imperceptible to the naked eye. While the more evident factors include gravity and wind, there are an additional 28 peculiar and intricate variables at play, both of physical and environmental nature, that collectively shape the bullet's path in flight.
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 vary in their shapes, weights, and sizes, and these factors significantly impact their performance during flight. Achieving the ideal mass and shape for a specific caliber is essential to maximize velocity and energy. Precision ammunition offers a wide range of tailored recipes to craft the perfect bullet for precise distances, specific targets, and barrel lengths.
The caliber of the bullet has a profound impact on its flight trajectory, akin to the intricacies of rocket science. In precision shooting communities, reloaders rely on extensive data, referencing various sources, conducting tests, and consulting charts to fine-tune their bullets. This meticulous process allows them to create customized rounds that effectively counter the diverse external ballistic effects encountered in shooting scenarios.
Read More: Best Long Range Precision Calibers
Bullet Design Profile Matters
The profile or shape of a bullet significantly influences its spin and gyroscopic stability, crucial for accurate flight. Enhanced aerodynamics in bullet profiles lead to improved performance, preserving velocity, stability, and preventing tumbling during flight.
Bullets featuring a low-drag profile with a boat tail design excel in long-range scenarios, increasing kinetic efficiency and speed. Low-drag coefficient bullets experience slower deceleration, reducing the lateral drift caused by crosswinds.
For those seeking utmost precision, experimentation with different powder charges becomes an additional step to further optimize bullet performance.
2. What Causes Bullet Yaw
Yaw, the sideways motion or tumbling of a bullet during flight, occurs due to aerodynamic drag as the bullet moves through the air. This effect is common to all flying objects and is more pronounced at higher bullet velocities. Effective control of yaw is essential for precise shooting.
The key to managing spin lies in the type of gun barrel used and the speed at which the bullet exits the barrel. The barrel's twist rate plays a crucial role in influencing bullet spin and stability during its flight. A faster bullet spin, facilitated by the appropriate barrel twist rate, reduces the likelihood of yaw, contributing to improved accuracy and performance downrange.
3. Does A Hot Bullet Affect Performance?
Indeed, the temperature of the bullet plays a role in external ballistic effects, but accurately measuring the bullet's temperature during flight is a complex task. Typically, shooters focus on measuring the barrel's temperature rather than the bullet's.
When a round is fired, rapid pressure buildup occurs inside the chamber, and this pressure rapidly dissipates once the bullet exits the barrel. The bullet is slightly larger in diameter than the barrel, creating a snug fit that allows pressure to build up when the gunpowder ignites.
This pressurized environment propels the bullet out of the barrel, contributing to its flight dynamics. While bullet temperature is challenging to measure directly in real-time, shooters often consider the temperature effects on the entire system, including the barrel and ammunition, to make adjustments for external ballistic factors.
4. Why Does A Bullet Have To Spin
Bullet spin is indeed essential for maintaining stability during flight. As the bullet spins, it remains on a straight and accurate trajectory, aligning itself with its intended flight path. The rifling in the barrel imparts this spin to the bullet as it exits.
This concept is analogous to throwing a football. When a football is thrown with a proper spin, it stabilizes its flight and maintains a straight path. In both cases, spin plays a crucial role in achieving accuracy and predictability in the projectile's trajectory.
5. Precession
Bullet precession, known as gyroscopic precession, results in post-barrel wobbling, causing instability. Bullet weight distribution, similar to balancing car wheels with lead weights to prevent wobbling at certain speeds, contributes to this phenomenon.
The bullet experiences analogous precession-induced wobbling during flight, as bullet spin generates angular momentum, leading to intriguing external ballistics behavior.
6. Nutation
Bullet tips may lack concentricity, resulting in imperfect roundness. During precession, bullets exhibit nutation-induced swerving, causing small circular motions at the tip.
Bullet instability is influenced by both bullet length and spin rate. Longer bullets spin faster and face greater challenges in achieving stability.
Consequently, the bullet's profile and angle of attack play crucial roles in ensuring 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.
Bullet length plays a pivotal role, especially in low-drag bullets designed for optimal long-range ballistic performance. The distance between the center of pressure and the center of gravity dictates the angle of attack, enabling the bullet to assume an angle between its spin axis and flight path, a phenomenon known as the Poisson effect (with the bullet's nose tilted above the trajectory line).
This effect resembles the flight of a football. Aerodynamic forces exert pressure on the center of pressure, causing the bullet to tumble. However, by increasing the bullet's spin rate sufficiently, the gyroscopic effect counteracts tumbling, akin to how a rocket stabilizes its flight 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.
Clockwise spin causes rightward drift, while counterclockwise spin leads to leftward drift, with most rifle barrels featuring a right twist.
Spin drift arises from the bullet's gyroscopic motion during flight, deflecting it in the direction of its spin. The magnitude of this deflection depends on factors such as bullet length, flight duration, and spin rate.
As the bullet travels, its wobbling diminishes over distance until it achieves stability, causing its longitudinal axis to no longer align with its flight direction—this is known as the Yaw of Repose.
9. Barrel Twist Rate Affects Bullet Differently
The barrel's twist rate significantly influences this phenomenon, with most barrels featuring a right twist in their rifling. The twist rate signifies the speed of spin; for instance, a 1/10 twist rate means one complete revolution of bullet spin occurs over a span of 10 inches.
Heavier bullets require a tighter twist rate to achieve stability during flight, and consistency in the twist rate is essential for optimal accuracy. Various barrel lengths and calibers demand specific twist rates to perform at their best.
It's worth noting that firing the same bullet in barrels with different twist rates may lead to slightly varied velocities, but the difference is generally negligible.
10. Gain Twist - Variable Twist Rate
A rifling twist rate that becomes progressively tighter along the length of the barrel is known as a gain twist. This means that the barrel may start with a 1/12 twist rate and gradually transition to a 1/8 twist rate.
While there is some evidence suggesting that gain twist barrels may potentially enhance velocity with specific caliber bullets, proving this effect conclusively can be challenging.
11. Magnus Effect
A spinning projectile induces pressure variations by altering the orientation of its axis of symmetry relative to its flight axis. As the bullet travels through the air, it generates a high-pressure area due to the airflow around it, which exerts pressure on the bullet's center of pressure.
The Magnus force, always perpendicular to the lateral wind force affecting the bullet, also influences the bullet's center of pressure, leading to yawing motion.
While the bullet's center of gravity aligns with the trajectory line, any disparity with the center of pressure results in a torque, causing the bullet to deviate from its intended path.
12. Yaw of Repose
The Magnus effect initiates yaw, altering aerodynamic stability, ballistic coefficient, and travel direction while contributing significantly to spin drift.
A stable bullet exhibits a yaw angle, a result of aerodynamic drag forces acting on both the center of pressure and the center of mass.
The yaw angle corresponds to the yaw of repose, determined by the direction of the bullet's spin.
13. Distance
As a bullet traverses through the air over an extended distance, it becomes increasingly susceptible to various environmental factors, including gyroscopic drift, wind, gravity, air density, temperature, and kinetic energy.
With greater distance to the target, the bullet's trajectory begins to deviate, emphasizing the importance of accurate holdover adjustments and choosing the appropriate caliber for long-range shots. Minor imperfections that may go unnoticed at shorter ranges can magnify significantly when shooting over extended distances.
Top External Ballistic Effects - Environmental
14. Crosswind & Aerodynamic Jump
Crosswinds can blow from either the left to the right or from the right to the left, perpendicular to the bullet's flight path. A left crosswind will push the bullet further to the right, while a right crosswind will do the opposite.
The longer the wind affects the bullet during its journey, the greater the drift it experiences, resulting in a change in the bullet's point of impact (POI).
Aerodynamic jump refers to the bullet's vertical impact shift when encountering a crosswind, although it's a less commonly used term in practical long-range ballistics. It signifies the vertical deflection caused by a crosswind.
Specifically, wind coming from the right will cause the bullet to hit high, and vice versa.
For cost-effective solutions, using a Dope Disc Windage Solver is recommended, as many ballistic programs do not account for aerodynamic jump. While this deviation is subtle, it plays a crucial role in ballistic calibration.
15. Tailwind and Headwind
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
Vertical winds, whether upward or downward, can indeed impact a bullet's trajectory, causing it to rise or drop accordingly. This phenomenon is especially prevalent in mountainous regions where terrain-induced vertical winds occur.
In real-world scenarios, bullets can encounter a combination of horizontal, vertical, and crosswinds. Each component of the wind exerts its influence on the bullet's flight path.
These various external ballistic effects are interconnected within a system. A comprehensive understanding of each element's behavior contributes to greater precision in long-range shooting. By combining the right rifle with suitable bullets, you can optimize your setup for accurate long-distance shots.
17. Do Bullets Actually Rise?
Indeed, the bullet's trajectory follows a parabolic arc, and when observing its flight downrange through a spotting scope, it can create the illusion of a slight rise followed by a fall.
Several factors contribute to this effect. One reason is that the line of sight through the scope is slightly above the bore, causing the apparent rise. Additionally, some rifle barrels may be slanted slightly upward to compensate for the initial drop of the bullet, which further adds to this visual perception. However, it's important to note that the bullet itself will never rise above the axis of the rifle barrel during its flight.
18. Bullet Kinetic Energy
The explosion of gunpowder indeed generates substantial pressure to propel the bullet from the rifle barrel, and the energy of the bullet is frequently measured in foot-pounds.
Kinetic energy is a critical factor in a bullet's performance because it determines its terminal ballistic effectiveness. In simpler terms, it signifies the bullet's stopping power and its ability to incapacitate or knock down a target. The range of kinetic energy can vary significantly, from as low as 135 foot-pounds for a .22LR to as high as 5000 foot-pounds for a .338 Lapua Magnum, illustrating the wide range of capabilities across different calibers and loads.
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.
The subtle spinning motion of the Earth generates the Coriolis effect, causing slight deflection of bullets. While it's generally negligible at medium distances, beyond 1000 yards or more, it can influence your accuracy by shifting the point of impact. In the Northern hemisphere, a flying object tends to deflect to the right.
The horizontal component of the bullet trajectory is dependent on the shooter's latitude. The deflection occurs to the left or right relative to the shooting direction, not east or west.
The Coriolis effect is most significant at the poles and nonexistent at the equator of the Earth in terms of horizontal deflection.
Regarding the vertical component of the Coriolis effect, it depends on the shooting direction relative to the Earth's rotation. When shooting east, the point of impact will be higher, while shooting west will result in a lower point of impact. Shooting north or south generally neglects this effect.
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.
In long-range shooting, the angle at which you shoot has a significant impact on external ballistic effects, particularly when the shot angle exceeds 45 degrees. To compensate for bullet drop, the question of whether to aim higher or lower when shooting downhill arises.
When shooting downhill, you should aim slightly below the target. This compensates for the shooting angle, as the bullet drop won't be as significant relative to the point of aim due to the downward trajectory.
The same principle applies when shooting uphill. To account for the angle, you should hold the sight below the target.
This technique is referred to as high-angle shooting. Professional shooters utilize tools such as shooting angle calculators to determine the angle-modified range. Additionally, they use rangefinders and cosine angle indicators to enhance precision and accuracy in such shooting scenarios.
Click here to read more about high angle shooting.
21. Temperature Effect On Rifle Accuracy
Temperature indeed plays a crucial role in controlling air density, which directly affects the drag experienced by a bullet during its flight. In colder temperatures, the air is denser, leading to increased drag, which can impede the bullet's performance.
Conversely, warmer temperatures result in thinner air, reducing drag on the bullet. This decreased drag allows the bullet to exhibit improved performance, traveling farther than it would in colder weather conditions. Understanding and accounting for these temperature-related effects is essential for precision shooting and long-range ballistics.
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 can indeed have an impact on external ballistic accuracy, particularly in relation to a bullet's spin performance. Air with high humidity is denser than dry air because water molecules weigh less than dry air molecules. This effect can be further influenced by cold temperatures.
Higher humidity leads to thicker air, which can make it more challenging for the bullet to travel through the humid environment. As air density increases due to higher humidity, the bullet encounters more resistance, causing it to slow down.
However, you correctly noted that one of the most influential factors affecting accuracy is the density altitude. Density altitude takes into account variables like temperature, humidity, and altitude, providing a comprehensive measure of air density that has a significant impact on a bullet's flight characteristics. Understanding and accounting for density altitude is crucial for achieving precision in long-range shooting.
23. Density Altitude
Altitude does indeed affect a bullet's performance. At higher altitudes, the air is thinner, which results in reduced air resistance, allowing the bullet to travel faster and experience less drop over its trajectory. The key factor in determining the amount of bullet drop is its velocity - the faster it flies, the less it drops.
While density altitude is an important environmental variable that impacts bullet speed, it's essential to recognize that factors like wind and gravity have more significant influences on a bullet's trajectory than altitude alone. Wind can significantly alter the bullet's path, and gravity is a constant force that always affects the bullet's trajectory. Understanding and accounting for these factors is critical for precise long-range shooting.
24. Rifle Barrel Temperature
In precision shooting, the term "cold bore shot" is commonly used to evaluate a bullet's accuracy when fired from a barrel that has not been heated by previous shots. The idea is to assess how a rifle performs when starting with a cold barrel compared to one that has been fired multiple times and has reached a higher temperature.
Barrel temperature can influence factors like barrel harmonics, bullet velocity, and point of impact, which can affect accuracy. Maintaining consistent barrel temperature or understanding how temperature changes affect the rifle's performance is essential for precision shooting.
25. Bullet Weight Matters
The mass of the bullet, when combined with sufficient muzzle velocity, delivers the necessary energy to effectively incapacitate or knock down a target. The bullet's weight contributes to its penetration and stopping power.
The center of gravity of the bullet serves as a reference point that follows the ballistic trajectory. The optimal distance between the center of pressure and the center of gravity plays a vital role in determining the flight performance of the bullet, affecting its stability and accuracy downrange.
26. Bullet Profile & Ballistic Coefficient
B.C = Drag of the standard projectile / Drag of test projectile, quantifies the ratio of air drag.
In simpler terms, higher B.C values indicate that bullets are more aerodynamically efficient, allowing them to maintain velocity better. This means that slightly heavier bullets with the same shape can often outperform lighter bullets at the same initial muzzle velocity due to their improved ballistic coefficients.
Understanding and selecting bullets with optimal B.C values is essential for achieving better long-range performance and accuracy.
27. Boat Tail Bullet Means Low Drag
Boat tail bullets are designed to improve stability and accuracy in flight. Their tapered tail, resembling the stern of a boat, reduces drag and increases the bullet's ballistic coefficient (B.C).
The higher B.C of boat tail bullets enhances their ability to overcome air resistance and wind deflection. Additionally, the streamlined shape reduces drag, leading to improved spin performance during flight. All these factors contribute to better accuracy and long-range performance, making boat tail bullets a popular choice for precision shooting and hunting applications.
28. Ballistic Tip Affect Accuracy?
Full Metal Jacket (FMJ): FMJ bullets have a soft lead core encased in a harder metal (typically copper or brass). They are known for their streamlined shape, which enhances their accuracy and long-range performance. FMJ bullets are designed to maintain their shape upon impact and are less likely to expand or deform. They are often used in military applications and target shooting.
Ballistic Tip Bullets: Ballistic tip bullets, on the other hand, also feature a streamlined shape similar to FMJ bullets but come with a plastic or polymer tip. This tip is designed to promote controlled expansion upon impact, making them suitable for hunting applications. The combination of accuracy and expansion makes ballistic tip bullets a popular choice for hunters looking for both accuracy and terminal ballistics.
External Ballistics Reference Books
Applied Ballistics For Long Range Shooting 3rd Edition
This book delves into advanced external ballistics, explaining the science of accuracy in a reader-friendly manner. Champion shooter Bryan Litz simplifies complex concepts in 432 pages of highly recommended content. Click here to buy on Amazon
Ballistic Performance of Rifle Bullets 3rd Edition
This book provides performance data for over 724 modern rifle bullets, making it a must-read for dedicated precision shooting enthusiasts. Authored by Bryan Litz, it offers unparalleled content not available online.
Best online reference
TiborasaurusRex is my top pick. If you don’t like to read, then head over to his YouTube channel.
He shines the light on the undiscovered area of precision shooting to the public. He explains thoroughly and his videos have great educational content.
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