Knowledge base: bullet external ballistics

Here we collect some useful definitions for long-range shooting beginners, enthusiasts, and HitX users.

Altitude

It's a linear measure of the potential difference between a point on the Earth’s surface and at the point of elevation counting (the starting point adopted in your region). In the starting point, the height is taken equal to zero.

Ballistic Coefficient

BC (or drug function) is one of the most important parameters of a bullet, which affects its resistance to air during the flight. It is calculated using special formulas. There are two main BC standards G1 and G7. The G1 model works more accurately for bullets with a cylindrical (flat-based) body. The G7 model will work more accurately with bullets in which the diameter of the body decreases to the rear of the bullet (boat-tailed). Use the standard of BC specified by the manufacturer.

Flat-based bullet body vs Boat-tailed bullet body

Caliber

It is the diameter of the bullet, the same as bullet diameter. The most used calibers in the long-range shooting are: .308 Win, .223 Rem, 6.5mm Creedmoor, 6mm Creedmoor, .338 Lapua Magnum, .300 Win Mag, etc. 

Click value

The clicks value of horizontal or vertical adjustments at the knob of the optical sighting device. Typically, this value is 1/4 MOA (less often 1/8 MOA) or 0.1 MRAD. As a rule, these values ​​are indicated on the adjustment knob or in the optical scope manual documentation. 

optical scope adjustment knobs

Coriolis force

The force which indicates the movement of the target along with the rotation of the Earth during the flight of the bullet. Coriolis force in ballistics is usually used when shooting long ranges. For its calculation, it is important to include the geolocation of the shooter and from which side of the world it is positioning to the target.

Derivation force

The force arising from the bullet rotation, that is, due to the gyroscopic effect and the Magnus effect. Affects the calculation of the trajectory of a bullet flight.

First focal plane (FFP) reticle placement

If the reticle of the optical scope is in the first focal plane – its size will always be the same regarding the shooter but will vary regarding the target at any magnification. Reticle mark values for this kind of optical scopes are always true at any magnification.

MOA

Minute of Angle (or Angular Minute) is a unit of measure for the angle equal to 1/60 degree and corresponds to 1.047 inches at a distance of 100 yards. 

MRAD

Also MIL or milliradian – this angle unit corresponds to 1/10 meter per 100 meters of distance. Thus, 0.1 MIL (MRAD) reticle mark in the optical scope will be 1 centimeter at a distance of 100 meters.

Muzzle velocity

The initial velocity of a bullet is the speed of a bullet when fired, measured at the muzzle of a rifle. The average value is usually specified by the bullet manufacturer. More accurate data can be obtained by self-measurement using special measuring equipment. 

Scope elevation

Also scope height – the distance between the aiming axis of the mounted scope and the axis of the rifle barrel. Typically it depends on the optical scope diameter and mount system.

Aiming axis and Shooting axis

Scope reticle placement

It's the location of the reticle in the first (FFP or First Focal Plane) or second (SFP or Second Focal Plane) focal planes. This parameter affects how the reticle will behave when changing the magnification of the optical sight device.

Second focal plane (SFP) raticle placement

If the reticle of the scope is in the second focal plane – its size will vary regarding the shooter but will stay the same regarding the target at any magnification. Reticle mark values for this kind of optical scopes are always true at certain magnification. Information about the reticle placement is usually specified by the manufacturer.

Target elevation

Also slope angle – the difference between the elevation of the shooter and the target, displayed in angular units (degrees).

The imperial or English system of measures

It is used in the United Kingdom (although the metric system has been used officially since 1995), the USA, Myanmar, partly in Canada, Liberia. The main units of measurement are miles, yards, feet, and grains.

The metric system (SI)

It is the general name of the international decimal system of units based on the use on the meter, second, kilogram, kelvin, ampere, mole, and candela. Currently, the metric system is officially adopted in all countries of the world, except the USA, Liberia, and Myanmar (Burma).

Twist

Barrel twist is a very important characteristic for ballistics showing the distance at which the twisted rifling of the barrel makes one full rotation. The rifling of the barrel can be twisted either to the right (less often) and to the left (more often). It affects bullet flight stability. This characteristic can be determined visually or by looking at the specs in the manual documentation. 

Zeroing range

Also zero distance – the distance at which optical scope has zero adjustments.