The speed of a bullet is a common point of curiosity, but there is no single number that provides a definitive answer. The speed at which a bullet travels varies dramatically, changing from the instant it leaves the barrel until it strikes a target. This initial speed, known as muzzle velocity, depends on a complex interaction of physics and engineering.
Translating Bullet Speed From FPS to MPH
The ballistics industry measures bullet speed using feet per second (FPS) because it provides a precise figure for technical calculations. To translate this figure into miles per hour (MPH), the FPS value is multiplied by a conversion factor of approximately 0.682. This calculation helps visualize the immense speeds involved.
The full spectrum of bullet velocities is vast, ranging from the slowest handgun rounds to the fastest rifle cartridges. At the low end, some heavy, subsonic pistol bullets travel at around 500 FPS, which is roughly 340 MPH. High-power rifle projectiles routinely exceed 4,000 FPS, translating to speeds greater than 2,700 MPH.
Factors That Determine Muzzle Velocity
The initial speed of a bullet is determined by three main internal factors that occur within the firearm’s chamber and barrel. The primary accelerating force comes from the propellant, often called gunpowder, which is ignited by the firing pin. The amount and specific chemical composition of this propellant dictates the volume and speed of the expanding gases that push the projectile forward.
The length of the barrel provides the necessary distance for these expanding gases to act on the bullet. A longer barrel allows the gas pressure more time to build and accelerate the projectile, resulting in a higher muzzle velocity. Conversely, a shorter barrel cuts off the acceleration phase earlier, meaning the bullet leaves the muzzle at a lower speed.
The third factor is the projectile’s weight, or mass, which is often measured in grains. A heavier bullet requires more force to achieve the same acceleration as a lighter one, given the same amount of propellant. For example, when comparing two bullets of the same caliber, the lighter bullet will generally exit the barrel at a higher velocity.
Comparing Common Caliber Speeds
The speed of sound, or Mach 1, provides a useful benchmark for bullet velocities, sitting at approximately 761 MPH at sea level under standard conditions. Projectiles traveling slower than this speed are considered subsonic, while those traveling faster are supersonic and produce a distinctive sonic boom. For instance, a heavy 9mm handgun round might travel at 1,000 FPS, or 682 MPH, which is well into the subsonic range.
A common rifle cartridge like the .223 Remington is significantly faster, achieving velocities around 3,000 FPS. This speed is equivalent to approximately 2,045 MPH, or nearly Mach 2.7, highlighting the massive difference between handgun and rifle performance. The fastest common cartridges, such as the .220 Swift, can push small bullets to speeds exceeding 4,000 FPS, converting to more than 2,727 MPH.
How Air Resistance Slows a Bullet Down
The moment a bullet leaves the muzzle, it begins to lose speed, as the powerful accelerating force of the propellant gas is instantly replaced by external forces. The most significant factor slowing the bullet is air resistance, also known as drag. This force acts against the bullet’s direction of travel and is proportional to the square of the projectile’s velocity, meaning it is most intense when the bullet is moving fastest.
The ability of a bullet to overcome air resistance is quantified by its ballistic coefficient, which relates its mass, diameter, and aerodynamic shape. A bullet with a higher ballistic coefficient will maintain its velocity over a longer distance more effectively than one with a lower rating. Furthermore, the constant downward pull of gravity causes the bullet to immediately begin dropping from its line of fire, forcing it to follow a downward parabolic path. Consequently, the speed upon impact, or terminal velocity, is always substantially less than the initial muzzle velocity.