Sectional Density

This Sectional Density calculator computes the factor used in determining a bullet's ability to penetrate upon impact.

INSTRUCTIONS: Choose units and enter the following:

• (d) Diameter of Bullet
• (M) Mass of Bullet 

Sectional Density (SD):  The value is returned as a real number

The Math / Science

Sectional density (SD) is a measure used in ballistics to describe the ratio of a bullet's mass to its cross-sectional area. It is an important parameter in understanding a bullet's performance, particularly in terms of penetration through various media. In simpler terms, sectional density is the mass of the bullet divided by the square of its diameter. The resulting value provides information about the bullet's ability to penetrate through air and, more importantly, its ability to penetrate through a target. Higher sectional density generally indicates better penetration potential, as the mass is concentrated in a smaller frontal area. Bullets with higher sectional density are often preferred for tasks that require deep penetration, such as hunting larger game. However, it's important to consider other factors like bullet design, construction, and velocity in conjunction with sectional density for a comprehensive understanding of a bullet's performance.

The formula for sectional density is 

            SD = M/d²

where:

• SD is the sectional density
• M is the mass of the projectile or bullet in pounds
• d is the diameter of the projectile or bullet in inches.

This calculator automatically converts grains, grams and other mass/weight units to pounds, and millimeters and other length units to inches.  The resulting answer is provided with no units.

In firearms, sectional density helps in understanding how efficiently a bullet can penetrate a target. Bullets with higher sectional density (greater weight relative to their diameter) tend to penetrate better because they concentrate their mass in a smaller frontal area. A bullet's sectional density affects its ability to overcome air resistance, maintain velocity, and retain energy as it travels towards the target. Sectional density is considered by shooters, hunters, and ammunition manufacturers when selecting bullets for specific purposes. It can influence decisions on the type of ammunition to use for hunting different game animals or for achieving specific ballistic performance, such as deeper penetration or better terminal ballistics.

For example, a heavier bullet of the same caliber (diameter) generally has a higher sectional density compared to a lighter bullet. Given the same velocity, a bullet with higher sectional density is likely to penetrate deeper into the target due to its increased mass concentrated in a smaller area.

The ballistic sectional density is used to compute the ballistic coefficient of a bullet.  The ballistic sectional density is also used in computation of the penetration characteristics of terminal ballistics.

Caliber

The term "caliber" when referring to firearms and ammunition is a measure of the internal diameter of a gun barrel or the diameter of a bullet. It is usually expressed in inches or millimeters. The diameter of the bullet or the bore of the gun is a critical factor in determining the appropriate ammunition for a particular firearm.

For example, if a firearm has a caliber of .45 inches, it means that the internal diameter of the gun barrel is approximately 0.45 inches. In the context of ammunition, a bullet designed for a .45 caliber firearm would also have a diameter of around 0.45 inches.

It's important to note that caliber is just one aspect of the overall cartridge specification. The complete cartridge information typically includes the caliber, cartridge length, and sometimes additional information, such as the specific type of ammunition or load. Understanding the correct caliber is crucial for selecting the right ammunition for a firearm to ensure proper function and safety.

There are numerous bullet calibers used in firearms, and they vary widely depending on the type of firearm and its intended purpose. Here are some common bullet calibers:

• .22 LR (Long Rifle): Common for rimfire rifles and handguns, often used for plinking and small-game hunting.
• 9mm Parabellum (9mm Luger): Widely used for handguns, including many military and police firearms.
• .45 ACP (Automatic Colt Pistol): Commonly used in handguns, particularly in large-frame pistols.
• .380 ACP (9mm Short): Used in compact and subcompact handguns for self-defense.
• .40 S&W (Smith & Wesson): Popular for law enforcement and self-defense applications, often found in semi-automatic pistols.
• .38 Special: Common in revolvers, used for both self-defense and target shooting.
• .357 Magnum: Similar to .38 Special but more powerful, often used in revolvers.
• .223 Remington / 5.56x45mm NATO: Common rifle cartridges used in civilian and military rifles.
• .308 Winchester / 7.62x51mm NATO: Used in a variety of rifles, including hunting and military rifles.
• 12 gauge and 20 gauge: Common shotgun gauges, used for various applications such as hunting and sport shooting.

These are just a few examples, and there are many other calibers for different firearms and purposes. When selecting ammunition for a firearm, it's crucial to use the correct caliber and type specified by the firearm's manufacturer. Mixing incompatible calibers can be dangerous and may result in malfunctions or accidents.

Firearm Calculators

• Sectional Density: Computes the factor used in the computation of the ballistic coefficient called sectional density.
• Bullet Ballistic Coefficient: Computes factor that represents a bullet's ability to overcome air resistance in flight.
• Ballistic Coefficient from Bullet's Mass, Diameter and Form Factor: Estimates the ballistic coefficient from the mass, diameter and form factor
• Miller Twist Rule: Computes the optimal barrel twist rate for stabilizing a bullet's flight given the bullet's diameter, length and mass.
• Taylor Knock-out Factor: Calculates a factor indicating the power of a round,.
• Greenhill Formula for Optimal Rifling Twist Rate: calculate the optimal barrel twist rate for stabilizing a bullet's flight given the bullet's diameter, length, specific gravity and velocity. 
• Bullet Flight Range: Computes the maximum range (horizontal distance) traveled by a bullet based on the muzzle velocity, elevation angle and shooter height.
• Muzzle Energy of a Projectile: Calculates the kinetic energy in a bullet immediately after leaving the barrel given the bullet's mass and velocity.
• Recoil Velocity of a Gun: Computes the velocity at which a gun will move in the opposite direction in relation to the projectile that it fired.
• Cost per Round: Computes cost per round of ammunition based on the cost of a container of cartridges and the number of rounds in the container.
• Rifle Sight Correction Angles: Computes the Minutes of Angle corrections for rifle sights.
• Shotgun Shell Reloading Cost: Computes the cost to reload used shotgun shells (hulls) with powder, shot, wads and primers.
• Shotgun Shell Loading Cost: Computes the cost to load new pre-primed shells (hulls) with power, shot and wads. 
• Metal Ball Weight: Computes the mass (weight) of a spherical metal (e.g., steel) ball based on the size (diameter).
• Metal Cylinder Weight: Computes the mass (weight) of a metal cylinder based on the size (diameter and length).

References

• BALLISTIC COEFFICIENT MEASUREMENTS