Shot grouping in the context of "Shotshell"

A shotgun cartridge, shotshell, or shell is a type of rimmed, cylindrical (straight-walled) ammunition used specifically in shotguns. It is typically loaded with numerous small, spherical sub-projectiles called shot. Shotguns typically use a smoothbore barrel with a tapered constriction at the muzzle to regulate the extent of scattering.

Some cartridges contain a single solid projectile known as a slug (sometimes fired through a rifled slug barrel). The casing usually consists of a paper or plastic tube with a metallic base containing the primer. The shot charge is typically contained by wadding inside the case. The caliber of the cartridge is known as its gauge.

Indirect fire is shooting a projectile without relying on a direct line of sight between the gun and its target, in contrast to the case of direct fire. Aiming of indirect fire is instead performed by predicting a parabolic ballistic trajectory via calculation of the azimuth and inclination, and may include calibrating the aim by observer feedback about the actual point of impact of the preceding shot and thus readjusting to new firing angles for subsequent shots.

Indirect fire is the principal method of long-range artillery fire support, both from land and naval platforms. Due to the projectile's longer flight time (which exposes it more to deflectional factors such as drag and crosswind), curved trajectory, and the far and often obstacled "beyond-visual-range" nature of the targets, indirect fires are inherently harder to aim accurately than direct fires, resulting in a more unpredictable external ballistics and thus a much more scattered shot grouping. This, coupled with the significant blast distance of the explosive ordnance and their shrapnels, translates to a much higher risk of collateral damages and friendly fires, especially when firing danger-close.

Shooting targets are objects in various forms and shapes that are used for pistol, rifle, shotgun and other shooting sports, as well as in darts, target archery, crossbow shooting and other non-firearm related sports. The center is often called the bullseye. Targets can for instance be made of paper, "self healing" rubber or steel. There are also electronic targets that electronically can provide the shooter with precise feedback of the shot placement.

A milliradian (SI-symbol mrad, sometimes also abbreviated mil or mils) is an SI derived unit for angular measurement which is defined as a thousandth of a radian (0.001 radian). Milliradians are used in adjustment of firearm sights by adjusting the angle of the sight compared to the barrel (up, down, left, or right). Milliradians are also used for comparing shot groupings, or to compare the difficulty of hitting different sized shooting targets at different distances. When using a scope with both mrad adjustment and a reticle with mrad markings (called an "mrad/mrad scope"), the shooter can use the reticle as a ruler to count the number of mrads a shot was off-target, which directly translates to the sight adjustment needed to hit the target with a follow-up shot. Optics with mrad markings in the reticle can also be used to make a range estimation of a known size target, or vice versa, to determine a target size if the distance is known, a practice called "milling".

Milliradians are generally used for very small angles, which allows for very accurate mathematical approximations to more easily calculate with direct proportions, back and forth between the angular separation observed in an optic, linear subtension on target, and range. In such applications it is useful to use a unit for target size that is a thousandth of the unit for range, for instance by using the metric units millimeters for target size and meters for range. This coincides with the definition of the milliradian where the arc length is defined as ⁠1/1,000⁠ of the radius. A common adjustment value in firearm sights is 1 cm at 100 meters which equals ⁠10 mm/100 m⁠ = ⁠1/10⁠ mrad.