The measurement of a bullet’s speed is a core concept in ballistics, the science dedicated to the motion of projectiles. Velocity determines a projectile’s trajectory, energy upon impact, and overall performance. The unit of Mach provides context by comparing the projectile’s velocity to the speed of sound. This comparison is important because crossing the sound barrier introduces aerodynamic effects that change a bullet’s flight path and stability.
Understanding Mach and the Sound Barrier
Mach is a dimensionless unit expressing the ratio of an object’s speed to the speed of sound in the surrounding medium. Mach 1 is exactly the speed of sound, which is not fixed but changes based on atmospheric conditions. At sea level in dry air at 68 degrees Fahrenheit (20 degrees Celsius), the speed of sound is approximately 1,125 feet per second (343 meters per second). Therefore, a bullet traveling at 1,125 feet per second is moving at Mach 1.
The speed of sound decreases with lower temperatures, meaning a projectile’s Mach number varies with altitude and weather. When a bullet travels at or above Mach 1, it breaks the “sound barrier.” This generates a continuous pressure wave, known as a shock wave, which trails behind the projectile. The resulting sonic boom is the noise heard as the bullet passes, often described as a sharp “crack.”
Typical Bullet Speeds by Classification
Bullet speeds are categorized into three regimes related to the speed of sound.
Subsonic
This classification includes projectiles traveling below Mach 1, typically under 1,100 feet per second. Many heavier handgun rounds, such as the .45 ACP or specialized 9mm loads, are designed to be subsonic to avoid the sonic crack. This speed range is preferred when using a suppressor, as the loudest component of the shot is eliminated.
Transonic
Speeds between 1,100 and 1,300 feet per second, hovering around Mach 1, are considered Transonic. This zone is difficult in ballistics because the airflow over the bullet’s surface is a mix of subsonic and supersonic regions. The shifting shock waves in the transonic range can negatively affect the projectile’s stability and accuracy. Many standard handgun rounds, like 9mm or .40 S&W, fall into this category.
Supersonic
This is the fastest and most common classification for long-range projectiles, where the speed exceeds Mach 1. Most centerfire rifle cartridges, such as the .223 Remington or .308 Winchester, are highly supersonic. These rifle bullets achieve speeds between Mach 2 and Mach 3, corresponding to muzzle velocities of roughly 2,500 to over 3,000 feet per second. High-velocity cartridges, like the .220 Swift, can approach Mach 4, reaching over 4,000 feet per second.
Factors Determining a Bullet’s Mach Speed
A bullet’s final Mach speed is a direct result of the interaction between its components and the firearm’s design.
Propellant
The type and amount of propellant are the primary driving forces behind the speed. Modern smokeless powder burns rapidly, releasing gas to build pressure behind the bullet. Slower-burning powder requires more time to convert to gas, which must be matched to the barrel length.
Bullet Weight and Design
The bullet’s weight and design influence the achievable Mach speed. For a given powder charge pressure, a lighter bullet accelerates to a higher velocity than a heavier one. The projectile’s shape and construction determine how efficiently it resists the air. Sleeker, more aerodynamic designs maintain a higher Mach speed downrange. A bullet with a smaller cross-sectional area also experiences a higher force-to-area ratio.
Barrel Length
The barrel length plays a role in determining the muzzle velocity. A longer barrel provides a greater distance for the expanding gases to push the projectile, allowing for a longer period of acceleration. Pressure increases the bullet’s speed until it exits the muzzle. This is why rifles, which have long barrels, generally produce higher Mach speeds than handguns.
A higher Mach speed translates directly into greater kinetic energy, the measure of the projectile’s potential to do work upon impact. This increased energy allows supersonic rifle rounds to deliver their effect at greater distances than slower handgun counterparts. Ballistic performance is tied to the Mach number.