Internal ballistics

[2][3] Interior ballistics can be considered in three time periods:[4] The burning firearm propellant produces energy in the form of hot gases that raise the chamber pressure which applies a force on the base of the projectile, causing it to accelerate.

The size and shape of the grains can increase or decrease the relative surface area, and change the burning rate significantly.

The size and shape of the propellant grains can increase or decrease the relative surface area, and change the burn rate significantly.

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.

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.

)[1] "Caseless ammunition" incorporates propellant cast as a single solid grain with the priming compound placed in a hollow at the base and the bullet attached to the front.

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.

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.

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.

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.

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.

For these reasons, when reliable feeding is more important than accuracy, such as with military rifles, longer cases with shallower shoulder angles are favored.

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,[17] 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.

At velocities over 460 m/s (1,500 ft/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 760 m/s (2,500 ft/s), and a common solution is to impregnate the surface of the bullet with molybdenum disulfide lubricant.

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 40 grains (2.6 g) can provide over 1,000 newtons (220 lbf) of resistance due to inertia.

Propellants are matched to firearm strength, chamber volume and barrel length; and bullet material, weight and dimensions.

[19] The rate of gas generation is proportional to the surface area of burning propellant grains in accordance with Piobert's Law.

[21] Accelerating gas generation from fast burning propellants may rapidly create a destructively high pressure spike before bullet movement increases reaction volume.

The downward push, on the other hand, helps counteract the rotation imparted by the fact that most firearms have the barrel mounted above the center of gravity.

The action-shooting handguns redirect all the energy up to counteract the rotation of the recoil, and make following shots faster by leaving the gun on target.

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 companies that use polygonal rifling claim greater accuracy, lower friction, and less lead and/or copper buildup in the barrel.

The muzzle must allow the gas to escape the barrel symmetrically; any asymmetry will cause an uneven pressure on the base of the bullet, which will disrupt its flight.

The muzzle end of the barrel is called the "crown", and it is usually either beveled or recessed to protect it from bumps or scratches that might affect accuracy.