Internal Ballistics

IGNITION

Priming methods

The first step to firing a firearm of any sort is igniting the propellant. The earliest firearms were Cannon s, which were simple closed tubes. There was a small aperture, the "touchhole", drilled in the closed end of the tube, leading down to the main Powder charge. This hole was filled with finely ground powder, which was then ignited with a hot Ember or Torch . With the advent of hand-held firearms, this became an undesirable way of firing the gun. Holding a burning stick, while trying to carefully pour a charge of black powder down a barrel, is a good way to get maimed or killed. Also, trying to hold the gun with one hand, aiming at the target, and looking for the touchhole, so you could put the burning stick against it with the other hand, was not conducive to any degree of accuracy.

External priming

Matchlock:
The first attempt to make the process of firing a small arm easier was the "matchlock". The matchlock incorporated a "lock" (so called, because of its resemblance to door locks of the day) that was actuated by a Trigger . The lock was a simple lever which pivoted, when pulled, and lowered the match down to the touchhole. The match was a slow burning Fuse made of plant fibers that were soaked in a solution of Nitrates , Charcoal , and Sulphur , and dried. This was ignited before the Gun might be needed, and it would slowly burn, keeping a hot ember at the burning end. After the gun was loaded, and the touchhole primed with powder, the burning tip of the match was positioned, so that the lock would bring it into contact with the touchhole. To fire the gun, it was aimed, and the trigger pulled. This brought the match down to the touchhole, igniting the powder. The slow burning match could be kept going, with careful attention, for long periods of time, and the use of the lock mechanism made accurate fire (within the limits of the gun) possible.

Wheel-lock:
The next revolution in ignition technology was the "wheel-lock". It used a spring-loaded, Serrate d Steel Wheel which rubbed against a piece of Iron Pyrite . There was a key which was used to wind the wheel, and put the spring under tension. Once tensioned, the wheel was held in place by a trigger. When the trigger was pulled, the serrated edge of the steel rubbed against the pyrite, generating Spark s. These sparks were directed into a pan, called the "flashpan", filled with loose powder which lead into the touchhole. The flashpan was usually covered by a spring-loaded cover that would slide out of the way when the trigger was pulled, exposing the powder to the sparks. The wheel-lock was a major Innovation — since it did not rely on burning material as a source of Heat , it could be loaded, and kept loaded for extended periods of time. The covered flashpan also gave the gun some ability to withstand bad weather. Wind, rain, and wet weather would render a matchlock useless, but a wheel-lock that was loaded, and waterproofed with a bit of Grease around the flashpan, could be fired under most conditions.

Flintlock:
The wheel-lock enjoyed only a brief period of popularity before being superseded by a simpler, more robust design. The "flintlock", like the wheel-lock, used a flashpan and a spark to ignite the powder. As the name implies, the flintlock used Flint , rather than iron pyrite. The flint was held in a spring-loaded arm, called the "cock". The cock rotated through about a 90 degree Arc , and was held in the tensioned, or "cocked" position by a trigger. Usually, flintlocks would lock the cock in two positions. The "half-cock" position held the cock halfway back, and used a deep Notch , so that pulling the trigger would not release the cock. This was a safety position, used when loading, and when storing or carrying a loaded flintlock. The "full-cock" position held the cock all the way back, and was the position from which the gun was fired. The "frizzen" was the other half of the flintlock ignition system. It served as both a flashpan cover, and a steel striking surface for the flint. The frizzen was hinged, and spring-loaded, so that it would lock in the open or closed position. When closed, the striking surface was positioned so that the flint would strike at the proper angle to generate a spark. The striking flint would also open the frizzen, exposing the flashpan to the spark. The flintlock mechanism was simpler, and stronger than the wheel-lock, and the flint and steel provided a good, reliable source of ignition. The flintlock remained in military service for over 200 years, and flintlocks are still made today for historical re-enactments, and for hunters who enjoy the additional challenge that the flintlock provides.

Caplock:
The next major leap in ignition technology was the invention of the chemical primer, or "cap", and the mechanism which used it, called the "caplock". The caplock appeared just before the American Civil War , and was quickly adopted by both sides as it was even simpler and more reliable than the flintlock. The main reason the caplock was so quickly adopted was its similarity to the flintlock. The flashpan and frizzen were removed, and replaced by a "nipple" which the cap fit onto. The cock was replaced by a "hammer", which also had half-cock and full-cock positions for the same reasons. When fired, the hammer would hit the cap, crushing it onto the nipple. The Percussion Cap was a thin metal cup that had in it, a small quantity of pressure-sensitive explosive. When crushed, the explosive would detonate, sending a stream of hot Gas down a hole in the nipple, and into the touchhole of the gun. In the process of firing, the cap generally split open, and would fall off, when the hammer was moved to half-cock position for loading. The caplock system worked well, and is still the preferred method of ignition for Hunter s and recreational shooters who use Muzzle -loading arms.

Internal priming

Chemical Primer s, advanced Metallurgy and Manufacturing techniques all came together in the 1800s to create an entirely new class of firearm — the cartridge arm. Flintlock and caplock shooters had long carried their Ammunition in Paper Cartridge s, which served to hold a measured charge of powder, and a Bullet in one convenient package (the paper also served to seal the bullet in the bore). Still, the source of ignition was separate. With the advent of chemical primers, it was not long before all sorts of systems were invented, with a multitude of different ways of combining bullet, powder, and primer into one package which could quickly be loaded from the Breech of the firearm. The three systems which have survived the test of time are the rimfire, the Berdan primer, and the Boxer primer.

Rimfire:
Rimfire cartridges use a thin Brass case with a bulge, or rim, around the back end. This rim is filled, during manufacture, with an impact-sensitive primer. In the wet state, the primer is stable; a pellet of wet primer is placed in the shell, and simply spun out to the full extremes of the rim. (For more on the exact process and one set of chemical compounds that have been used successfully, see , a 1932 Remington Arms patent by James E. Burns.) In the dry state, the primer within the rim becomes impact-sensitive. When the rim is then crushed by the hammer or firing pin, the primer detonates and ignites the powder charge. Rimfire cartridges are single-use — after firing, they cannot practically be reloaded. Also, since the rim must be thin enough to be easily crushed, the pressures generated in the case are limited by the strength of this thin rim. Rimfire cartridges used to be available in Caliber s up to .44, but all except the small .22 caliber rounds died out. The .22 long Rifle (which is also fired in Pistol s) is the most popular recreational caliber, because it is inexpensive, quiet, and has very low Recoil . The most inexpensive brands can be bought for less than US$0.02 per round in boxes of 500, and even Olympic class Ammunition is around US$0.20 per round. Additionally, in 2002 another rimfire cartridge, the .17 HMR , was introduced. The .17 HMR is essentially a .22 magnum rimfire cartridge necked down to accept a .17 caliber bullet, and is used as a flat-shooting, light duty varmint round.

Berdan Primer:
The remaining types of priming, Berdan and Boxer, are both considered "centerfire", to differentiate them from the rimfire rounds. Centerfire priming methods are interchangeable; the same firearm can fire both Berdan- and Boxer-primed rounds.

Berdan primers are named after their American inventor, Hiram Berdan of New York who invented his first variation of the Berdan primer and patented it on March 20, 1866, in . A small copper cylinder formed the shell of the cartridge, and the primer cap was pressed into the ''outside'' end of the cartridge opposite the bullet from the outside. In the end of the cartridge beneath the primer cap was a single vent-hole, as well as a small "teat-like projection" or point fashioned from the case, later to be known as an anvil, upon which to provide a hard surface behind the primer cap such that the firing pin would have a hard surface against which to crush the primer and ignite the propellant. This system worked well, allowing the option of installing a cap just before use of the propellant-loaded cartridge, as well as permitting re-loading the cartridge for re-use. Difficulties arose in practice because pressing in the cap from the outside tended to cause a swelling of the copper cartridge shell, preventing the reliable seating of the cartridge in the chamber of the firearm. Berdan's solution was to change to brass shells, and to further modify the process of installing the primer cap into the cartridge, as noted in his second Berdan Primer patent of September 29, 1869, in . Berdan primers have remained essentially the same functionally to the present day.

Berdan primers are similar to the caps used in the caplock system — being small metal cups, with pressure-sensitive explosive in them. Modern-day Berdan primers are pressed into the primer "pocket" of a Berdan-type cartridge case, where they fit slightly below, flush with the base of the case. Inside the primer pocket is a small bump, the "anvil", that rests against the center of the cup, and two small holes that allow flash from the primer to reach the interior of the case. Berdan cases are reusable, although the process is rather involved. The used primer must be removed, usually by Hydraulic pressure, or a lever that pulls the primer out of the bottom. A new primer is carefully seated against the anvil, and then gunpowder and a bullet are added. Berdan priming is used by nearly all militaries and most civilian manufacturers, with the ironic exception of the USA .

Boxer Primers:
Meanwhile, Edward M. Boxer , of the Royal Arsenal in Woolwich, England was working on a similar primer cap design for cartridges, patenting it in England on October 13, 1866, and subsequently receiving a U.S. patent for his design on June 29, 1869, in .

Boxer primers are similar to Berdan primers with one major change — the location of the anvil. In a Boxer primer, the anvil is a separate piece that sits in the primer cup. Because of this, the primer pocket has the flash-hole, centered. This makes little or no difference to the performance of the round, but it makes fired primers vastly easier to remove for re-loading. A thin metal rod is pushed through the mouth of the case, and it pushes the primer out. A new primer, anvil included, is then pressed into the case. Since the primer and anvil are sold as one part, the anvil depth must be correct for the primer that is being inserted, so that the primer does not ignite during loading (although priming is done as the first step, before the powder is added). This is the main reason, why Boxer priming is still popular in the USA, as there are a large number of shooters who reload their ammunition.

Boxer-primed ammunition is slightly more complex to manufacture, since the primer is in two parts, but the slight increase in initial cost is often more than equalized by the decreased cost of firing reloaded rounds, at least for users intending to reload rounds. However, in much military-surplus ammunition, Berdan-primed ammunition is often found to be more common, having been both cheaper and faster to produce for filling very-large orders intended for military use. Military-surplus Berdan-primed ammunition is also often corrosive or slightly-corrosive, whereas Boxer-primed ammunition is often non-corrosive, although assuming corrosive or non-corrosive characteristics on the basis of whether Berdan or Boxer primed is never fool-proof.

Sizes Of Primers:
Primers come in different sizes, based on the application. The types/sizes of primers are:

For both pistol and rifle: Small, Large, and Magnum .

The primer size is based on the primer pocket of the cartridge.

The primer explosive charge is based on the amount of ignition required by the cartridge design.

Electrical

Nearly all firearms today still use essentially the same technology that was developed for the caplock — a spring-loaded hammer or striker is used to crush the primer, which causes a small Detonation that ignites the powder charge. Since this system is mechanical, there is a small but significant lag — called the "lock time" — between the pull of the trigger, and the ignition of the cartridge. This is usually only a few hundredths of a second, but when you consider that the bullet leaves the barrel in a few thousandths of a second after ignition, the lock time is the major source of delay. When the shooter is trying to time a shot to fire between Heartbeat s, those hundredths of a second can mean the difference between a gold medal and no medal at all. Another place where lock time is critical is in the multi-barrel Gatling Gun s used by militaries, which have rates of fire, as high as 100 rounds per second. Many high-speed Gatling guns, and a growing number of civilian arms are switching to electrical primers. Electrical primers in civilian arms use a battery pack and a Capacitor to build up an Electrical Charge that is then sent through a Transformer to increase the voltage to 150 to 200 Volt s. This charge is then sent through the primer, where it ignites the explosive, and thus sets off the propellant charge. Larger bore military weapons use thousands of volts to create a spark which detonates the primer.

PROPELLANT

Black powder

''Main article: Black Powder ''

Black powder is a mix of Sulphur , Charcoal , and Potassium or Sodium Nitrate . Unlike smokeless propellants, it acts more like an explosive since its burn rate is not affected by Pressure . However, it is a very poor explosive because it has a very slow decomposition rate, and therefore a very low Brisance .

Nitrocellulose

Nitrocellulose is formed by the action of Nitric Acid on Cellulose fibers. It is a highly combustible Plastic that Deflagrates rapidly, when heat is applied. It also burns very cleanly, burning almost entirely to gaseous components at high Temperature s. The burning rate of nitrocellulose is dependent upon the pressure — a pile of uncontained nitrocellulose will burn slowly, with a high, bright Flame , but when placed in a high strength, sealed container, the same material will burn very quickly, bursting the container. Since nitrocellulose is a Plastic , it can be formed into many shapes of gunpowder, such as cylinders, tubes, balls, and Flake s. The size and shape of the powder grains can increase or decrease the relative Surface Area , and change the burn rate significantly. Additives and coatings can be added to the powder to further modify the burn rate. Normally, very fast powders are used for low-velocity Pistol s and Shotgun s, medium-rate powders for magnum pistols and light Rifle rounds, and slow powders for large-bore heavy rifle rounds.

Double base propellants

To further increase the energy of smokeless powder, Nitroglycerin can be added in amounts up to 50%. These powders are called "double base powders", and they have the same basic physical properties as single base powders. The nitrocellulose serves to desensitize the highly unstable nitroglycerin, and the nitroglycerin greatly increases the energy density of the resulting powder. Double base powders burn faster than single base powders of the same shape, and in general, the higher the nitroglycerin content of a powder, the faster the burn rate.

Solid propellants

A recent topic of research has been in the realm of "caseless cartridges". In a caseless cartridge, the propellant is cast as a single solid grain, with the priming compound placed in a hollow at the base, and the bullet glued to the front. Since the single propellant grain is so large (most smokeless powders have grain sizes around 1 mm, a caseless grain will be perhaps 7 mm diameter and 15 mm long), the relative burn rate must be much higher. To reach this rate of burning, caseless propellants often use moderated explosives, such as RDX .

While there is at least one experimental military rifle (the H&K G11), and one commercial rifle (made by Voere), that use caseless rounds, they are not having much success. The caseless ammunition is by necessity, not reloadable (a major disadvantage in civilian markets, where reloading is common), and the exposed propellant makes the rounds less rugged. Also, the case in a standard cartridge serves as a seal, keeping gas from escaping the Breech . Caseless arms must use a more complex self-sealing breech, which increases the design and manufacturing complexity. Another problem peculiar to auto-loading arms firing caseless rounds is the problem of rounds " Cooking Off ". This is caused by residual heat from the chamber, heating the round in the chamber to the point where it ignites, causing an unintentional discharge.

Belt-fed machine guns, designed for high volumes of fire, are designed to fire from an open bolt, which means that the round is not chambered until the trigger is pulled, and so there is no chance for the round to cook off before the operator is ready. Open-bolt designs are generally undesirable for anything but belt-fed machineguns and pistol-sized sub-machine guns. The reason is that the mass of the bolt, moving forward, causes the gun to lurch in reaction, which significantly reduces the accuracy of the gun. Since one of the motivating factors for the use of caseless rounds is to increase the rate of fire to the degree that several shots can be fired to the same point of aim, anything that reduces the accuracy of those first shots would be counterproductive. Cased ammunition serves as a heat sink to both carry heat away from the chamber after firing, and to cool the chamber when chambered, reducing the risk of cook off.

Load density and consistency

Load Density is the Percentage of the space in the cartridge case that is filled with powder. In general, loads close to 100% density (or even loads where seating the bullet in the case, compresses the powder) ignite and burn more consistently than lower density loads. In cartridges that survived from the black powder cartridge era (examples being .45 Colt, .45-70 Government), the case is much larger than needed to hold the maximum charge of high-density smokeless powder. This allows the powder to shift in the case, piling up near the front, or near the back of the case. This can cause significant variations in burning rate, as powder near the rear of the case will ignite rapidly, but powder near the front of the case will ignite slower. This change has less impact with fast powders. As such, high-capacity, low-density cartridges generally deliver best accuracy with the fastest appropriate powder, although this keeps the total energy low, due to the sharp, high pressure peak.

Magnum pistol cartridges reverse this power/accuracy tradeoff, by using lower density powders that give high load density, and a broad pressure curve. The downside is the increased recoil and muzzle blast from the high powder mass, and high muzzle pressure. The advantage is that the magnum pistol rounds generate accuracy, comparable to a good hunting rifle, and energy sufficient to take medium game at ranges out to 100 yards (100 meters) and beyond.

Most rifle cartridges have a high load density with the appropriate powders. Rifle cartridges tend to be bottlenecked, with a wide base narrowing down to a smaller diameter, to hold a light, high-velocity bullet. These cases are designed to hold a large charge of low-density powder, for an even broader pressure curve than a magnum pistol cartridge. These cases require the use of a long rifle barrel to extract their full efficiency, although they are also chambered in rifle-like pistols (single shot or bolt action) with barrels of 10 to 15 inches (25 to 38 cm).

One unusual phenomenon occurs when high density powders are used in large capacity rifle cases. Small charges of powder, unless held tightly near the rear of the case by Wadding , can apparently Detonate when ignited. The mechanism of this phenomenon is not well-known, and generally it is not encountered, except when loading very low velocity Subsonic rounds for rifles. These rounds generally have velocities of under 500 ft/s (195 m/s), and are used for indoor shooting, or Pest Control , where the power and muzzle blast of a full power round is not needed or desired.

CHAMBER

Straight vs bottleneck

Straight walled cases were the standard from the beginnings of cartridge arms. With the low burning speed of black powder, the best Efficiency was achieved with large, heavy bullets, so the bullet was the largest practical Diameter . The large diameter allowed a short, stable bullet with high weight, and the maximum practical bore Volume to extract the most energy possible in a given length barrel. There were a few cartridges that had long, shallow tapers, but these were generally an attempt to use an existing cartridge to fire a smaller bullet with a higher velocity and lower recoil. With the advent of Smokeless Powder s, it was possible to generate far higher velocities by using a slow smokeless powder in a large volume case, pushing a small, light bullet. The .30-30 Winchester was one of the first rounds to be designed to use smokeless powder, and it has a distinct shoulder that closely resembles modern cartridges, even though it dates back to the 1890s . Modern cases have shorter necks, sharper shoulder angles, and a rimless design which give better efficiency and feeding, but the .30-30 cartridge and the Winchester model 1894 rifle still account for more game in North America than any other rifle and cartridge combination.

Aspect ratio and consistency

When selecting a rifle cartridge for maximum accuracy, a short, fat cartridge will generally yield a higher efficiency and consistency than a long, thin cartridge (part of the reason for a bottle-necked design). The ideal shape would be something near spherical, but this would be impractical to build. Target- and Varmint - hunting rounds require the greatest accuracy, so they tend to be short and fat, with sharp shoulders on the case. These cartridges do have disadvantages, however. The fat rounds take up a lot of space in a Magazine , and the sharp shoulders do not feed easily out of a magazine, and into the chamber. For this reason, rounds that do not require the utmost accuracy, but do need to be chambered in repeating arms (such as military rifles), tend to have longer cases with shallower shoulder angles.

FRICTION AND INERTIA

Static friction and ignition

Since the burning rate of smokeless powder varies directly with the pressure, the initial pressure buildup has a significant effect on the final Velocity , especially in cartridges with fast powders. The Friction , holding the bullet in the case, determines how soon after ignition the bullet moves, and since the Motion of the bullet increases the volume and drops the pressure, a difference in friction can change the slope of the pressure curve. In general, a tight fit is desired, to the extent of Crimp ing the bullet into the case. In straight-walled rimless cases, such as the .45 ACP, an aggressive crimp is not possible, since the case is held in the chamber by the mouth of the case, but sizing the case to allow a tight Interference Fit with the bullet, can give the desired result.

Kinetic friction

The bullet must tightly fit the bore to seal the high pressure of the burning gun powder. This tight fit generates a large quantity of friction. The friction of the bullet in the bore does have a slight impact on the final velocity, but that is generally not much of a concern. Of greater concern is the heat that is generated, due to the friction. At velocities of about 1000 ft/s (390 m/s), Lead begins to melt, and deposit in the Bore . This lead build-up constricts the bore, increasing the pressure and decreasing the accuracy of subsequent rounds, and is difficult to scrub out, without damaging the bore. Rounds, used at velocities up to 1500 ft/s (585 m/s), can use Wax Lubricant s on the bullet to reduce lead build-up. At velocities over 1500 ft/s (585 m/s), nearly all bullets are jacketed in Copper , or a similar Alloy that is soft enough not to wear on the barrel, but melts at a high enough temperature to reduce build-up in the bore. Copper build-up does begin to occur in rounds that exceed 2500 ft/s (975 m/s), and a common solution is to impregnate the surface of the bullet with Molybdenum Disulfide Lubricant . This reduces copper build-up in the bore, and results in better long-term accuracy.

The role of inertia

In the first few inches (centimeters) of travel down the bore, the bullet reaches a significant percentage of its final velocity, even for high-capacity rifles, with slow burning powder. The Acceleration is on the order of tens of thousands of Gravities , so even a projectile as light as 40 grains (2.6 g), can provide hundreds of pounds (over 1000 N) of resistance, due to Inertia . Changes in bullet mass, therefore, have a huge impact on the pressure curves of smokeless powder cartridges, unlike black powder cartridges. This makes loading or reloading smokeless cartridges require high-precision equipment, and carefully-measured tables of load data for given cartridges, powders, and bullet weights.

PRESSURE

of a Simulation of the 5.56 mm NATO round, being fired from a 20-inch barrel. The horizontal Axis represents time, the vertical axis represents pressure (green line), bullet travel (red line), and bullet velocity (light blue line). The values shown at top are peak values]]

Pressure vs distance traveled

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