Glossary of Shooting Terms
Angle of incidence--in sighting, the angular displacement between the line of sight through the optical device and the bore line of
the firearm. An aspect of parallax, it is this angular displacement that allows the shooter to 'zero' at a specific distance for the
purpose of overcoming the effect of gravity on a projectile. When one 'zeroes' a firearm he is essentially causing the line of sight
and the trajectory of the projectile to intersect at that distance.
Berdan-primed vs Boxer-primed -- in cartridge manufacturing, there are two basic types of primers--those in which the anvil is
part of the cartridge case (the brass) and housed in the primer pocket, Berdan-primed; and those in which the anvil is part of the
primer itself, the Boxer-primed version.
Chasing the rifling—in hand-loading, target shooting jargon: seating a bullet further out of the case to adjust the overall cartridge
length for the purpose of putting the bullet ogive closer to the bore rifling; it becomes more necessary as the throat and the edges of
the lands and grooves begin to erode. The practice helps control bullet start pressure and bullet gap ‘jump.’
Copper units of pressure or CUP, and the related lead units of pressure or LUP—terms applied to pressure measurements used
in the field of internal ballistics for the estimation of chamber pressures in firearms. These terms were adopted by convention to
indicate that the pressure values were measured by copper crusher and lead crusher gauges respectively. In recent years, they have
been replaced by the adoption of more modern piezoelectric pressure gauges that more accurately measure chamber pressures and
generally give significantly higher pressure values. This nomenclature was adopted to avoid confusion and the potentially dangerous
interchange of pressure values and standards made by different types of pressure gauges. Pressure is a fundamental thermodynamic
parameter that is expressed in units of force divided by area. In the avoirdupois system, the units of pressure are pounds per square
inch and in the metric system, the units or pressure are newtons per square meter (Pascals). A chamber pressure measured with a
copper crusher gauge would be expressed as psi (CUP) in the English system or MPa (CUP) in the metric system.
Until the invention of measurement transducers in the 1960s, crusher guns were the only reliable method for estimating chamber
pressures. Transducers are also faster to use in practice, as they do not require the careful measuring of the copper or lead
cylinders after firing. One outcome from this transition to using measurement transducers is, for example, that a Speer reloading
manual from 1987 lists all SAAMI pressures in CUP, while current references list nearly all pressures in PSI.
No accurate conversion formulas are possible, for converting between true pressures and crusher indicated pressures, linearized
approximation conversion equations do exist over narrow ranges of pressures for estimating piezo pressure readings given crusher
pressures. For example, the SAAMI maximum pressure for the 7.62 x 51 mm is given as 52000 psi (CUP), or 62000 psi (430
MPa); the .45-70, on the other extreme, is listed as 28000 in both CUP and psi (190 MPa). SAAMI standards for a given cartridge
may be expressed in CUP units, LUP units, or in standard units of pressure (psi or MPa).
As a result, although most published load data—even in older manuals—is likely to be safe, it is best to rely on newer load data
expressed in “PSI”. As a safety measure, try to find a similar load either on line or in a recently published load data table.
declination tangent--in external ballistics, that point in a bullet's trajectory at which it crosses back over the line of sight after
having reached its highest point (inclination apex). A 5.56 bullet, for example, fired level and zeroed at 100 yards, will cross the line
of sight again at approximately 250 meters--its declination tangent.
free bore—the distance between the end of the chamber and the beginning of the barrel rifling. Too much free bore causes the
bullet to have to ‘jump’ a significant distance before starting to spin, which diminishes accuracy; too little free bore limits the length
of bullet that can be safely loaded into the case without causing chamber pressure problems.
inclination apex --in external ballistics, the highest point the bullet reaches in its trajectory; the bullet will continue to climb to that
apex until gravity takes over and brings it back down. On the backside of trajectory, after the bullet starts back down, the bullet will
strike the same spot on a target on level ground at a much greater distance (see declination tangent).
load efficiency—several parameters establish the efficiency of the load: powder burn rate, case capacity, powder bulk, amount by
dry weight, clean burn, velocity and pressure. In the most efficient load, the right powder burns the full barrel length, at the lowest
pressure, achieves the highest velocity at the lowest bulk and weight and burns completely. An efficient load achieves the
established ballistic objective with the least velocity, terminal ballistics and materiel necessary to do the job.
Meplat—from a French term for a flat surface, is the technical term for the flat tip on the nose of a bullet. The shape of the meplat
is important in determining how the bullet moves through the air. If its meplat is uneven, a bullet will not have an ideal flight
characteristic and will most likely not be of the expected weight for that type of round. Bullets that have similar-shaped meplats will
travel through the air in nearly identical fashion, making it easier to group shots together or hit a target multiple times. For this
reason, competitive shooters and snipers often trim their meplats to uniform shape.
Trimming the meplat also shortens the bullet, making it more susceptible to wind drift. So even though the grouping will be tighter,
the range will be slightly reduced.
O.A.L (“Over All Length”) [Also: C.O.L. Cartridge Overall Length; “Min. OAL”] The overall length of the cartridge, usually
recommended by the manufacturer and or reloading guide, establishes the recommended overall length of the cartridge after the
bullet is seated and crimped either as a maximum or minimum length. The reason for the recommendation is that a bullet seated too
deep in the case causes the case capacity to diminish and raises pressure—and, without a means to measure it, the pressure
becomes dangerously unpredictable.
Ogive -- that part of a projectile at which the bearing surface begins to curve away from the bore towards the bullet tip; the 'ogive
number is usually expressed in bullet diameters. The number 2 ogive of a .308 bullet then is .616 inches.
parallax -- in general, the difference in the apparent position of an object viewed along two different lines of sight. Viewing objects
with two eyes side by side, from two slightly different angles, establishes human depth perception. In optics, parallax manifests
itself as a greying around the edges of a scope; indicating that the eye and the line of sight through the scope are slightly off.
Parallax clears when the eye aligns properly with the scope's field of view.
shot start pressure—chamber pressure necessary to engrave the bullet into the rifling of the bore.
squib (or ‘squib load’)—usually considered a malfunction, a squib is a projectile that does not leave the barrel. Reducing the powder
charge by half or more will result in a squib. Reloading a squib intentionally is one way of ‘swaging,’ ‘slugging or ‘casting’ the
lands and grooves of a gun barrel; firing the squib and then tapping the bullet out with a cleaning rod or dowel will print the
lands and grooves without the usual damage or distortion of some kind of media used to ‘catch’it.
Sub-sonic--in general terms, 'slower than the speed of sound'; anything traveling the speed of sound or faster in open air produces a
sonic crack or 'sonic boom.' The speed of sound is slightly altered by elevation, temperature and humidity. Sound travels about 750
miles per hour (1080 feet per second) at sea level, 70 degrees Fahrenheit at a relative humidity of 30 percent. In external ballistics, a
bullet traveling at this speed or greater would make a sonic crack as it passed, followed shortly by the muzzle report. The muzzle
report is caused not by the bullet but the expanding gasses which exceed the speed of sound. Even if a load is reduced to a subsonic
speed, the gasses escaping behind it will still be hypersonic. This is why a suppressor is needed to dampen the muzzle report by
containing and releasing the gases more slowly.