The term “Mach” frequently appears when discussing bullet speed, offering a standardized way to understand its velocity. This measurement is a ratio comparing an object’s speed to the speed of sound in its surrounding medium, not a fixed speed. Understanding bullet speed in Mach terms clarifies its performance and associated phenomena, providing a universal benchmark for comparison across ammunition types and environmental conditions.
What Mach Means
Mach is a unit of speed relative to the speed of sound, with “Mach 1” representing the speed of sound itself. If an object travels at Mach 2, it moves at twice the speed of sound, and Mach 0.5 indicates half the speed of sound. This ratio is important because the speed of sound is not constant; it changes based on atmospheric conditions.
Air temperature is the primary factor influencing the speed of sound; it slows as temperature decreases. Altitude also plays a role because air temperature typically drops at higher elevations. Therefore, a bullet traveling at Mach 1 will have a different actual velocity depending on the temperature and altitude at which it is fired. At 68°F (20°C), Mach 1 in dry air is approximately 1,125 feet per second (767 miles per hour).
Factors Affecting Bullet Velocity
A bullet’s velocity is shaped by several design and environmental factors. The type and amount of propellant within a cartridge significantly influence speed; a greater powder charge leads to higher velocities by creating more pressure. The length of the firearm’s barrel also plays a role; a longer barrel allows propellant gases more time to expand and push the bullet, increasing speed up to a certain point.
Bullet weight is another important consideration. Lighter bullets often achieve higher muzzle velocities than heavier ones when fired from the same firearm, though heavier bullets can retain their velocity more effectively over longer distances. The aerodynamic design of the bullet impacts how efficiently it cuts through the air. A more aerodynamic bullet experiences less air resistance, maintaining its speed more effectively over its flight path. Air resistance constantly slows the bullet from the moment it leaves the barrel.
Common Bullet Mach Speeds
Bullet speeds vary considerably depending on the cartridge and firearm type. Handgun rounds generally exhibit lower velocities compared to rifle cartridges. Many common handgun bullets, such as those from a 9mm Luger, travel around 1,200 feet per second (fps), approximately Mach 1.0 to 1.1. Some handgun calibers, like the .45 ACP, have velocities below the speed of sound, around 800-950 fps, putting them in the subsonic range of Mach 0.7 to 0.8. High-performance handgun rounds, like the .357 Magnum, can exceed 1,300 fps, reaching Mach 1.2 or higher.
Rifle bullets generally achieve much higher Mach speeds due to greater propellant charges and longer barrels. Many popular rifle cartridges, such as the .223 Remington or 5.56x45mm NATO, achieve muzzle velocities ranging from 2,700 to over 3,200 fps, corresponding to Mach 2.4 to 2.8. The .308 Winchester falls into a similar range, with speeds around 2,600 to 3,000 fps (Mach 2.3-2.6). Some of the fastest rifle cartridges, like the .220 Swift, can reach speeds exceeding 4,000 fps, approaching Mach 3.5-3.7. Even rimfire ammunition now includes hyper-velocity loads that can exceed Mach 2.
Supersonic and Subsonic Bullet Flight
The distinction between supersonic and subsonic bullet flight has implications for both acoustics and ballistic performance. A bullet is supersonic when its speed exceeds Mach 1. A subsonic bullet travels slower than the speed of sound, staying below Mach 1.
When a bullet travels at supersonic speeds, it creates a “sonic boom,” often heard as a sharp “crack” or “ballistic crack.” This occurs because the bullet compresses the air in front of it, forming shock waves that continuously trail behind the projectile. This ballistic crack is distinct from the gun’s muzzle blast, which is why even suppressed firearms firing supersonic ammunition still produce a noticeable sound.
Subsonic ammunition avoids this ballistic crack because it does not break the sound barrier. This makes subsonic rounds quieter, especially when used with a sound suppressor, as only the muzzle blast needs to be mitigated. The trade-off for reduced noise is lower velocity, less kinetic energy, and more bullet drop over distance. Supersonic rounds offer flatter trajectories and greater energy transfer at impact, making them suitable for long-range shooting where noise is less of a concern.