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Internal ballistics (also interior ballistics), a subfield of ballistics, is the study of the propulsion of a projectile. In guns, internal ballistics covers the time from the propellant's ignition until the projectile exits the gun barrel. The study of internal ballistics is important to designers and users of firearms of all types, from small bore rifles and pistols, to high tech artillery. For rocket propelled projectiles, internal ballistics covers the period during which a rocket motor is providing thrust.

Internal ballistics was not scientifically based prior to the mid 1800s. Barrels and actions were built strong enough to survive a known overload (Proof test). Muzzle velocity was surmised from the distance the projectile traveled. In the 1800s test barrels began to be instrumented. Holes were drilled in the barrel and fitted with standardized steel pistons which exerted pressure which compressed standardized copper cylinders when the firearm discharged. The reduction in the copper cylinder length is used as an indication of peak pressure, known as "Copper Units of Pressure", or "CUP" for high pressure firearms. Similar standards were applied to firearms with lower peak pressures, typically common handguns, with test cylinder pellets made of more easily deformed lead cylinders, hence "Lead Units of Pressure", or "LUP". The measurement only indicated the maximum pressure that was reached at that point in the barrel. Piezoelectric strain gauges were introduced in the 1960's, allowing instantaneous pressures to be measured without destructive pressure ports. Instrumented projectiles were developed by the Army Research Laboratory that measures the pressure at the base of the projectile and acceleration.

Methods of igniting the propellant evolved over time. A small hole (a touch hole) was drilled into the breech, into which a propellant was then poured, and an external flame or spark applied (see matchlock and flintlock). Percussion caps were and self contained cartridges have primers that detonate after mechanical deformation, igniting the propellant.

Gunpowder (Black powder) is a finely ground, pressed and granulated mechanical pyrotechnic mixture of sulfur, charcoal, and potassium nitrate or sodium nitrate. It can be produced in a range of grain sizes. The size and shape of the grains can increase or decrease the relative surface area, and change the burning rate significantly. The burning rate of black powder is relatively insensitive to pressure, meaning it will burn quickly and predictably even without confinement, making it also suitable for use as a low explosive. It has a very slow decomposition rate, and therefore a very low brisance. It is not, in the strictest sense of the term, an explosive, but a "deflagrant", as it does not detonate but decomposes by deflagration due to its subsonic mechanism of flame front propagation.

Nitrocellulose or "guncotton" is formed by the action of nitric acid on cellulose fibers. It is a highly combustible fibrous material that deflagrates rapidly when heat is applied. It also burns very cleanly, burning almost entirely to gaseous components at high temperatures with little smoke or solid residue. Gelatinised nitrocellulose is a plastic, which can be formed into cylinders, tubes, balls, or flakes known as single base propellants. The size and shape of the propellant grains can increase or decrease the relative surface area, and change the burn rate significantly. Additives and coatings can be added to the propellant to further modify the burn rate. Normally, very fast powders are used for light bullet or low velocity pistols and shotguns, medium rate powders for magnum pistols and light rifle rounds, and slow powders for large bore heavy rifle rounds.

Nitroglycerin can be added to nitrocellulose to form "double base propellants". Nitrocellulose desensitizes nitroglycerin to prevent detonation in propellant sized grains, (see dynamite), and the nitroglycerin gelatinises the nitrocellulose and increases the energy. Double base powders burn faster than single base powders of the same shape, though not as cleanly, and burn rate increases with nitroglycerin content. In artillery, Ballistite or Cordite has been used in the form of rods, tubes, slotted tube, perforated cylinder or multi tubular; the geometry being chosen to provide the required burning characteristics. (Round balls or rods, for example, are "degressive burning" because their production of gas decreases with their surface area as the balls or rods burn smaller; thin flakes are "neutral burning," since they burn on their flat surfaces until the flake is completely consumed. The longitudally perforated or multi perforated cylinders used in large, long barreled rifles or cannon are "progressive burning;" the burning surface increases as the inside diameter of the holes enlarges, giving sustained burning and a long, continuous push on the projectile to produce higher velocity without increasing the peak pressure unduly. Progressive burning powder compensates somewhat for the pressure drop as the projectile accelerates down the bore and increases the volume behind it.) While there is at least one experimental military rifle (the H&K G11), and one commercial rifle (the Voere VEC 91), that use caseless rounds, they have met with little success. One other commercial rifle was the Daisy VL rifle made by the Daisy Air Rifle Co. and chambered for .22 caliber caseless ammunition that was ignited by a hot blast of compressed air from the lever used to compress a strong spring like for an air rifle. The caseless ammunition is of course not reloadable, since there is no casing left after firing the bullet, and the exposed propellant makes the rounds less durable. 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 unpleasant problem, common to all rapid firing arms but particularly problematic for those firing caseless rounds, is the problem of rounds "cooking off". This problem is caused by residual heat from the chamber heating the round in the chamber to the point where it ignites, causing an unintentional discharge. To minimize the risk of cartridge cook off, machineguns can be designed to fire from an open bolt, with the round not chambered until the trigger is pulled, and so there is no chance for the round to cook off before the operator is ready. Such weapons could use caseless ammunition effectively. Open bolt designs are generally undesirable for anything but machine guns; the mass of the bolt moving forward causes the gun to lurch in reaction, which significantly reduces the accuracy of the gun, which is generally not an issue for machinegun fire.

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 surviving from the black powder era (examples being .45 Colt, .45 70 Government), the case is much larger than is needed to hold the maximum charge of high density smokeless powder. This extra room allows the powder to shift in the case, piling up near the front or back of the case and potentially causing 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 later. This change has less impact with fast powders. 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, slower burning 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. 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).

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 powders, 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 odd, highly tapered 8 mm Lebel, made by necking down an older 11 mm black powder cartridge, was introduced in 1886, and it was soon followed by the 7.92×57mm Mauser and 7×57mm Mauser military rounds, and the commercial .30 30 Winchester, all of which were new designs built to use smokeless powder. All of these have a distinct shoulder that closely resembles modern cartridges, and with the exception of the Lebel they are still chambered in modern firearms even though the cartridges are over a century old.

When selecting a rifle cartridge for maximum accuracy, a short, fat cartridge with very little case taper may yield higher efficiency and more consistent velocity than a long, thin cartridge with a lot of case taper (part of the reason for a bottle necked design). Given current trends towards shorter and fatter cases, such as the new Winchester Super Short Magnum cartridges, it appears the ideal might be a case approaching spherical inside. Target and vermin hunting rounds require the greatest accuracy, so their cases tend to be short, fat, and nearly untapered with sharp shoulders on the case. Short, fat cases also allow short action weapons to be made lighter and stronger for the same level of performance. The trade off for this performance is fat rounds which take up more space in a magazine, sharp shoulders that do not feed as easily out of a magazine, and less reliable extraction of the spent round. For these reasons, when reliable feeding is more important than accuracy, such as with military rifles, longer cases with shallower shoulder angles are favored. There has been a long term trend however, even among military weapons, towards shorter, fatter cases. The current 7.62×51mm NATO case replacing the longer .30 06 Springfield is a good example, as is the new 6.5 Grendel cartridge designed to increase the performance of the AR 15 family of rifles and carbines. Nevertheless, there is significantly more to accuracy and cartridge lethality than the length and diameter of the case, and the 7.62×51mm NATO has a smaller case capacity than the .30 06 Springfield, reducing the amount of propellant that can be used, directly reducing the bullet weight and muzzle velocity combination that contributes to lethality, (as detailed in the published cartridge specifications linked herein for comparison). The 6.5 Grendel, on the other hand, is capable of firing a significantly heavier bullet (see link) than the 5.56 NATO out of the AR 15 family of weapons, with only a slight decrease in muzzle velocity, perhaps providing a more advantageous performance tradeoff.

Since the burning rate of smokeless powder varies directly with the pressure, the initial pressure buildup,(i. e. "the shot start pressure"), has a significant effect on the final velocity, especially in large cartridges with very fast powders and relatively light weight projectiles. In small caliber firearms, 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 crimping 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. In larger caliber firearms, the shot start pressure is often determined by the force required to initially engrave the projectile driving band into the start of the barrel rifling; smoothbore guns, which do not have rifling, achieve shot start pressure by initially driving the projectile into a "forcing cone" that provides resistance as it compresses the projectile obturation ring.

The bullet must tightly fit the bore to seal the high pressure of the burning gunpowder. This tight fit results in a large frictional force. 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 , 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 , can use wax lubricants on the bullet to reduce lead build up. At velocities over , 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 , 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. Large caliber projectiles also employ copper driving bands for rifled barrels for spin stabilized projectiles; however, fin stabilized projectiles fired from both rifle and smoothbore barrels, such as the APFSDS anti armor projectiles, employ nylon obturation rings that are sufficient to seal high pressure propellant gasses and also minimize in bore friction, providing a small boost to muzzle velocity.

In the first few 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 can provide over 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. The loading or reloading of smokeless cartridges thus requires high precision equipment, and carefully measured tables of load data for given cartridges, powders, and bullet weights.

Energy is imparted to the bullet in a firearm by the pressure of gases produced by burning propellant. While higher pressures produce higher velocities, pressure duration is also important. Peak pressure may represent only a small fraction of the time the bullet is accelerating. The entire duration of the bullet's travel through the barrel must be considered.

Energy is the ability to do work on an object. Work is force applied over a distance. The total energy imparted to a bullet is indicated by the area under a curve with the y axis being force (i. e., the pressure exerted on the base of the bullet multiplied by the area of the base of the bullet) and the x axis being distance. Increasing the energy of the bullet requires increasing the area under that curve, either by raising the pressure, or increasing the distance the bullet travels under pressure. Pressure is limited by the strength of the firearm, and duration is limited by barrel length.

Propellants are matched to firearm strength, chamber volume and barrel length; and bullet material, weight and dimensions. The rate of gas generation is proportional to the surface area of burning propellant grains in accordance with Piobert's Law. Smokeless propellant reactions occur in a series of zones or phases as the reaction proceeds from the surface into the solid. The deepest portion of the solid experiencing heat transfer melts and begins phase transition from solid to gas in a foam zone. The gaseous propellant decomposes into simpler molecules in a surrounding fizz zone. Endothermic transformations in the foam zone and fizz zone require energy initially provided by the primer and subsequently released in a luminous outer flame zone where the simpler gas molecules react to form conventional combustion products like steam and carbon monoxide. The heat transfer rate of smokeless propellants increases with pressure, resulting in the rate of gas generation from a given grain surface area increased at higher pressures. Accelerating gas generation from fast burning propellants may rapidly create a destructively high pressure spike before bullet movement increases reaction volume. Conversely, propellants designed for a minimum heat transfer pressure may cease decomposition into gaseous reactants if bullet movement decreases pressure before a slow burning propellant has been consumed. Unburned propellant grains may remain in the barrel if the energy releasing flame zone cannot be sustained in the resultant absence of gaseous reactants from the inner zones. (See muzzle brake for more on the disadvantages of muzzle brakes.) Powder to projectile weight ratio also touches on the subject of efficiency. In the case of the .22 250 Remington, more energy goes into propelling the powder gas than goes into propelling the bullet. The .22 250 pays for this by requiring a large case, with much powder, all for a fairly small gain in velocity and energy over other .22 caliber cartridges.

The defining characteristic of a revolver is the revolving cylinder, separate from the barrel, that contains the chambers. Revolvers typically have 5 to 10 chambers, and the first issue is ensuring consistency among the chambers, because if they aren't consistent then the point of impact will vary from chamber to chamber. The chambers must also align consistently with the barrel, so the bullet enters the barrel the same way from each chamber. The throat in a revolver is composed of two separate parts, the cylinder throat and the barrel throat. part of the cylinder and sized so that it is concentric to the chamber and very slightly over the bullet diameter. The cylinder gap the space between the cylinder and barrel must be wide enough to allow free rotation of the cylinder even when it becomes fouled with powder residue, but not so large that excessive gas is released. The forcing cone where the bullet is guided from the cylinder into the bore of the barrel should be deep enough to force the bullet into the bore without significant deformation. Unlike rifles, where the threaded portion of the barrel is in the chamber, revolver barrels threads surround the breech end of the bore. It is possible that the bore is compressed when the barrel is screwed into the frame. Cutting a longer forcing cone can relieve this "choke" point, as can lapping of the barrel after it is fitted to the frame.