Firearms Principles – Guns a tootin!

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1. General Definitions
2. Gun Types
3. Ammunition Components
4. Gun Components

1. General Definitions

Small Arms: Firearms, Cannon, and Guns in General

The term arm refers to a weapon that is hand held and can be transported by one person. Therefore a firearm is a gun intended to be used and supported by a single individual.

The small arms category includes pistols through cannon. Guns larger than cannon are used in tanks, ships, artillery pieces and so on. They often have bore diameters 3 inches (76mm) or larger. They can be referred to as "guns", but are not considered small arms. Mortars, which provide indirect fire and operate differently, are not considered guns. Some artillery pieces can provide either direct or indirect fire. They too are not described here. Gun can be used as a generic term to refer to the entire class of weapons described here or any members of the class.

Automatic and Semiautomatic Firearms

Automatic firearms load the next round of ammunition and in some cases fire the next round without user intervention beyond activating the trigger. Guns which continue to fire as long as the trigger is activated and ammunition is fed are fully automatic or machine guns.

Operationally, semiautomatic firearms are derived from automatic arms. They are a special category of automatic arms where the firing cycle is interrupted so that only one round is fired each time the trigger is activated and released. Because of this relationship, the term automatic firearm can refer to either automatic or semiautomatic, or to both as a general class. However to prevent confusion, the more specific term semiautomatic should be used to describe such arms, and automatic should only be used to describe fully-automatic arms such as machine guns and submachine guns.

Fully automatic firearms are not readily available in the United States. Private ownership of fully automatic firearms is possible but the requirements are fairly onerous. In many places such requirements make them essentially impossible to own. Therefore most firearms with a military appearance one might find at a U.S. gun store are semiautomatic.

Some fully automatic guns have operating modes where they fire a single shot or a burst of 2 or 3 shots. In the former mode they operate semi-automatically, firing one round for each trigger pull. In the latter mode they operate fully automatically, firing for example 3 rounds per trigger pull.

The purpose of burst firing is to increase hit probability. That is, more rounds for a given point of aim and trigger actuation increases the likelihood of a hit, assuming the point of aim can be held relatively steadily during firing. However, the mechanism which counts shots adds mechanical, training, and operational complexity. It should also be acknowledged that with appropriate training and practice, users of regular fully automatic guns (i.e., with no burst mode) can and should be able to fire short bursts through skillful trigger manipulation.

Caliber, Bore Diameter

In small arms, caliber refers to bore diameter, that is the width of projectile and corresponding opening in the barrel. A .50 caliber machine gun has a projectile width and barrel inner dimension close to one half inch in diameter. A 7.62mm caliber gun fires a projectile nominally 7.62mm in diameter. The actual bullet and barrel dimensions allow some room for rifling, bullet deformation, and so on within the limits of acceptable interference friction.

In naval guns caliber usually refers to the length of the barrel, described as a multiple of bore diameters. A fifty caliber four inch naval gun therefore has a four inch bore diameter and a barrel 200 inches long.

2. Gun Types

Guns can be divided into those that are fired hand-held and those that are braced against the shoulder for firing. The former are called pistols, while the latter are referred to generically as long guns.

Generally soldiers are issued shoulder-fired rifles. Military officers may carry pistols as backup defensive arms.

Pistol

Pistol can refer to the general class of hand-fired guns. A more specific use of the term "pistol" is for self-loading (semiautomatic), magazine-fed hand guns.

Another type of pistol is the revolver. Revolvers have a single barrel but use a rotating cylinder with multiple chambers to provide a repeating action. Because this operating method is fundamentally different from self-loading arms, operation of revolvers is not described further.

Pistol rounds are usually used at distances of less than 50 meters.

Rifle

A rifle is a shoulder-fired arm utilizing a relatively high-velocity round. They are capable of accurate fire out to a few hundred meters.

The term rifle derives from the rifling of the barrel. Rifling is typically helical ridges formed on the inside of the barrel through hammering or drawing. As the projectile engages the rifling it imparts spin. The spin gyroscopically stabilizes the flight of the projectile once it has left the barrel, giving it greater precision over longer distances. Rifled barrels were a major military advance starting before 1800.

Pistol barrels are usually rifled.

Carbine

A carbine is usually a shorter, lighter rifle. It can also refer to pistol-caliber, shoulder-fired light arms. Carbines were originally developed to improve the mobility of horse-riding cavalry soldiers.

Machine Gun

A machine gun is usually a rifle-caliber or larger, crew-served, belt-fed automatic firearm. Traditionally one rifle-caliber machine gun is deployed per platoon-sized (8 person) unit. Larger machine guns, such as the .50 caliber Browning M2 are used in vehicle- or tripod-mounted applications.

Submachine Gun

A submachine gun is a pistol-caliber, magazine-fed, individual automatic firearm. In military environments they are traditionally deployed in special operations commando units where mobility is key, or tank crews where space inside the vehicle is tight. In police environments they are deployed in counter-terrorist or special weapons units.

Since they usually have longer barrels than pistols submachine guns make more efficient use of pistol ammunition and are effective out to about 100 meters, though traditionally they are deployed at shorter distances. They are usually shoulder-fired for better accuracy.

Assault Rifle

A true assault rifle is a small, rifle-sized, fully-automatic arm chambered for an intermediate-power rifle round such as 7.62 x 39mm (Soviet) or 5.56 x 45mm (NATO). Current examples include the AK-47, AK-74, M16, and Steyr AUG. Note that true assault rifles are the fully automatic military versions, not the semi-automatic versions commercially available in the U.S.

The recently designed FN P90 Personal Defense Weapon falls somewhere between the submachine gun and assault rifle categories. It fires a small, light, 5.7 x 28mm rifle-caliber round but is about the size of a submachine gun. The P90 was designed for support troops other than assault infantry, who need full-sized assault rifles. The effectiveness of this new round in combat has not been fully determined.

Shotgun

A shotgun is a shoulder-fired arm. Most have barrels with smooth bores and fire loads of multiple lead or steel shot (balls individually or collectively called shot). Shotguns are found in hunting, target shooting, military, police, and self-defense applications. Shotguns can also fire slugs (a single large projectile) from smooth or rifled barrels. Shot is effective to about 40 meters or less. Slugs are effective out to about 100 meters or less. Shotguns are usually used at the shorter end of these ranges.

Cannon

Cannon are automatic guns of calibers 20mm to about 40mm, mounted on vehicles such as aircraft and armored vehicles.

3. Ammunition Components

Cartridge

Cartridge firearms have been in service since the mid-1800s. In earlier designs such as flint-lock or percussion cap arms, the propellant (powder) and the ignition mechanism were individual components which had to be separately loaded or were part of the arm itself.

Modern cartridges have a case which holds bullet, propellant and primer in a single self-contained system. This complete unit and the bullet fired from it are also referred to as a round or shell.

The cartridge is perhaps the single most important factor making modern firearms practical. The cartridge is a stand-alone module of mechanical integrity sealing sensitive chemical compounds from the external environment, and keeping together all the consumable components needed for firing one round. This key development enabled the significant advance of arms technology starting in the latter half of the 19th century.

Bullet

The bullet is the projectile driven down the barrel by the pressure of hot, burning propellant gases. Bullet design is a fairly specialized sub-field involving a much design and experimentation. Bullets must be aerodynamically stable and have low drag at their design velocities. They must be soft enough to allow some deformation in the rifling of the barrel. Bullets must have sufficient strength to withstand high acceleration during firing.

One of the topics not covered in this document is ballistics. Internal ballistics is the study of bullet and barrel performance while the bullet is in the barrel. External ballistics studies the bullet in free flight from muzzle to target. Terminal ballistics is the study of the effects of the projectile in the target medium. Targets studied usually include armor plating, sheet metal, flesh, or flesh simulants such as ballistic gelatin. See the Bibliography for some ballistics references.

It was the .50 caliber Browning rifle bullet which helped shape the first successful manned supersonic aircraft (Chuck Yaeger's Bell X-1) since it was one of the few objects known to be aerodynamically stable at three times the speed of sound.

Case

Cartridge cases are usually made of brass or steel, though some shotgun cases are made mostly or entirely of plastic. As described above the case is the carrier which holds the propellant, primer and bullet securely. This makes it practical to transport and deploy the components as a single module, greatly simplifying logistics and use. To re-iterate, the important principle was to group the relevant components as a self-contained functional unit.

The most important role of the case after feeding is the sealing of the breech. During firing the case expands outward against the barrel chamber providing a gas-tight seal and preventing the high pressure gases from entering the rest of the arm. Such a release could case a failure hazardous to the user, since usually only the combined case/chamber/breech system has sufficient strength to withstand the high pressures generated. Since the case expands under great pressure against the chamber, considerable force is required to overcome friction in extracting the fired case from the chamber.

Primer

The modern cartridge's percussion primer was a major improvement in starting the ignition chain. Previously a spark from a flint, heat from a smoldering match (a cotton cord), or an externally placed percussion cap provided the source of ignition. The modern primer is cup-shaped and contains percussion (impact or pressure) sensitive explosive compounds. When struck, the primer expels hot burning particles onto the propellant (powder) contained in the cartridge.

In modern rifle and pistol rounds the primer is press fit and sometimes crimped into the back of the cartridge, where the striker can reach it. Cannon rounds are usually electrically fired. That is, the primer is an electrical device with two insulated, usually concentric terminals which is ignited through electrical current. In rimfire rounds, the primer compound is in the fold at the back of case which also forms the extraction rim. The striker impacts the rim, detonating the primer material.

Propellant (Powder)

The first part of the ignition chain is the primer. When struck, the primer detonates (it is a primary explosive rather than a propellant) and begins to ignite the propellant (powder). The powder then burns at a controlled rate appropriate to the particular bore diameter, projectile mass, barrel length, etc. Note that powder is not a primary explosive, so in normal operation it burns relatively gradually rather than detonating. The burning propellant generates high pressure gases which accelerate the projectile down the barrel. Smokeless powder is usually composed of double base (twice nitrated) compounds and binders (glues) to hold grain shape.

Modern powders are extruded in the shape of rods or discs and come in a variety of sizes which together with chemical composition affect the burning rate. Smaller-grained, fast-burning propellants are generally used in higher velocity applications and larger-grained, slower-burning used in lower velocity rounds. Matching of burning rates to bore diameters and projectile masses requires careful calculation and measurement of pressure versus time. This is important to keep within the safe pressure limits of the barrel and rest of the system. Such development can be aided by using piezoelectric cells or strain gauges feeding computerized data acquisition systems. Once calibration procedures are established, these measurement systems are more efficient and simpler to use than old-style mechanical copper crushers.

4. Gun Components

Barrel

The barrel is a tube which the bullet is projected. The pressure of gases driving a rifle bullet can reach about fifty thousand pounds per square inch (3333 bar) and temperature 3500 degrees Fahrenheit (2200 C) in the barrel. These rise and fall during the few milliseconds (thousandths of seconds) when the projectile is in the barrel. So both thermal and mechanical stresses affect the barrel. Formulas which describe gas pressure and projectile position and velocity have been determined experimentally, for example LeDuc's method. Pistol and shotgun rounds operate at considerably lower pressures than rifle rounds at approximately one third.
As described above, barrels are usually rifled since the spin imparted by the rifling stabilizes the projectile in flight, decreasing wind deflection, and increasing aerodynamic stability. Exceptions include shotguns which fire a collection of projectiles with each shot, and anti-armor guns which fire light, high-velocity sub-caliber rounds. The latter are usually fired from tanks with smooth bore barrels (barrels with no rifling).

The latter are usually surrounded by a sabot while in the barrel. The increased bore diameter occupied by the sabot means pressure is applied over a larger surface area, resulting in a higher velocity. The sabot falls away after the projectile leaves the barrel. The smaller projectile also has less frontal area to be affected by aerodynamic drag and travels more efficiently to target. Saboted slugs and bullets are sometimes used in rifles, shotguns, and black powder small arms.

Chamber

The chamber is the portion of the barrel or barrel extension which supports the cartridge case while it is in firing position.

Trigger

The trigger is the user interface to the firing assembly. It can be activated by finger pressure, or it can be an electro-mechanical device. In some systems the entire firing chain is electrical, for example in most 20mm and larger cannon and a few .22 rimfire competition rifles. In those systems the trigger is a switch or electrical relay.

Sear

The sear is the portion of the trigger mechanism which directly holds and releases the bolt or striker. It interfaces directly or indirectly with the trigger.

Striker (Firing Pin)

The striker is usually a small rod or hammer which impacts the primer of the cartridge, setting off its percussion-sensitive charge and beginning the propellant ignition chain. The striker assembly sometimes consists of multiple moving parts such as a hammer which hits a transfer bar or firing pin. Strikers are sometimes implemented as firing pins. The more generic term striker is generally used here.

Receiver

The receiver is the body or frame of the gun to which the barrel, ammunition feeding devices, stocks or handles attach, and in which bolt operates. The bolt often rides on rails, rods, or recesses in the receiver. The receiver is sometimes divided into separate assemblies to facilitate cleaning, stoppage clearing, or other operational issues. The receiver and operating parts inside are also called the action.
The receiver can be made from stamped sheet metal, cast and/or machined metal, high-technology plastics, or combinations thereof. One of the most successful applications of a plastic receiver is the frame of the Glock pistol. The frame has carbide (very hard metal) guides molded into it. The Glock's slide (bolt) is conventional tool steel with a high-tech subsurface finish.

Bolt (Breech)

The bolt or breech constrains the cartridge in such a way that high pressure gases generated upon firing are kept in the chambered case and barrel. This allows pressure to fall to safe levels before the action is opened to load the next round or stop firing. The Operating Systems section describes various ways this can happen.

Bolt Carrier

Especially in designs which use rotary locking, the bolt and bolt carrier can be separate objects. The bolt engages locking lugs in the receiver or chamber and the bolt carrier holds the bolt. Different methods of accomplishing this are described in the next Functions section. The bolt carrier is usually larger and more massive than the bolt.


Source: http://www.rkba.org/guns/principles/definitions/index.html