Firearm propellants are a specialized type ofpropellant used to discharge aprojectile (typically abullet,slug, orpellets) through thebarrel of afirearm. Mixtures of differentchemical substances are often used to control the rate of gas release, or preventdecomposition of the propellant prior to use. Short-barrel firearms such as handguns necessitate faster-burning propellants to obtain sufficientmuzzle energy, whilelong guns typically use slower-burning propellants. The pressure relationships between propellantchemical reactions and bullet response are described asinternal ballistics.
Although all firearm propellants are generally called powder,[1] the termgunpowder originally described mixtures ofcharcoal andsulfur withpotassium nitrate as anoxidizing agent.[2]: 133, 137 By the 20th century these early propellants were largely replaced bysmokeless powder ofnitrocellulose or similarlynitratedorganic compounds.[3]: 287–330
The oldest gun propellant wasblack powder, alow explosive made from a mixture of sulfur, charcoal, and potassium nitrate. It was invented in China during the 9th century as one ofFour Great Inventions, and still remains in occasional use as asolid propellant forantique firearms.[citation needed] Availability of black powder allowed invention of firearms about 1300AD. While a mixture of charcoal and potassium nitrate will react without sulfur, sulfur makes the mixture easier to ignite. Early difficulties obtaining pure potassium nitrate caused historical variation in the relative proportions of the three ingredients.[3]: 28–40 Similar propellants have been formulated mixing sulfur and carbonaceous materials with other oxidizing chemicals includingsodium nitrate,ammonium nitrate, or variouschlorates; but they proved less satisfactory for use in firearms.[2]: 137–138
Modern firearm propellants tend to be smokeless powders based on nitrocellulose or similarly nitrated organic compounds, first invented in the late 19th century as a cleaner and better-performing replacement for black powder. Modern smokeless powder may be corned into smallspherical balls, orextruded into cylinders or strips with manycross-sectional shapes usingsolvents such asether, which can be cut into short ("flakes") or long pieces ("cords").[4]: 28 [5]: 41
Black powder produces gas at a predictable rate unaffected by pressure, while the gas production rate of smokeless powder increases with increasing pressure.[6] The possibility of runaway pressures caused smokeless powder to destroy many firearms designed for black powder and required much more precise measurement of propellant charges. Bulk powders were intended to offer advantages of smokeless powder for use in firearms designed for black powder. Charges of bulk powder are loaded in the samevolume appropriate for black powder. Early formulations including partially nitratedcellulose with potassium nitrate orbarium nitrate were more successful in shotguns than in rifles.[3]: 287–289 Nitrocellulose produces greater volumes of gas per volume of solid than black powder does, so nitrocellulose bulk powders were less dense than later smokeless powders. Nitrocellulose bulk powders designed for early straight-sided black powder rifle cartridges like the.32-40 and.38-55 werefriable and easily crumbled. The increased surface area of crumbled grains could produce unsafe pressures through faster gas production.[2]: 152, 159 Nitrocellulose bulk powders have been discontinued in favor of bulk powder formulations likePyrodex.[4]: 27
The performance characteristics of a propellant are greatly influenced by itsgrain size and shape, because thespecific surface area influences theburn rate, which in turn influences the rate of pressurization. While otherpowders may be formed by crushing or pulverizing solids into very small pieces,[7][8][9] firearm propellants are typically manufactured in grains of geometric shapes to physically control rate of gas production in accordance withPiobert's law.[10]Shotgun andhandgun propellants may be flakes, whileImproved Military Rifle propellants were extruded as shortcylindrical tubes, andball propellants are smallspheres.[11]
Many propellants useexothermic reactions to generate gaseouscarbon monoxide,nitrogen andsteam while raisingtemperatures to increasepressure.[4]: 28 The propellant reaction is initiated by aprimer.[2]: 52 Pressure initially increases as the propellant is converted to gas, but starts to decrease as the bulletaccelerates down the barrel to create a larger volume for the expanding gas. The acceleratingforce difference betweenchamber pressure behind the bullet and exterior pressure is initially reduced bystatic friction to move the bullet out of thecartridge casing into the barrel and then bysliding friction as the bullet moves down the barrel.[12] Pressure insmall arms typically peaks between one-half and onemillisecond after ignition.Handgun propellants may reach peak pressure before the bullet leaves the cartridge case, while rifle propellants may move a bullet several inches before reaching peak pressure.[4]: 125 The motion and behavior characteristics of projectiles while under the influence of the gases produced by the propellant is known asinternal ballistics.[13]
Due to the relatively short distance a gun barrel can offer, thesealed acceleration time is very limited and only a small proportion of the total energy generated by the propellant combustion will get transferred to the projectile. The residual energy in the propellant gases getsdissipated to the surrounding in the form of heat,vibration/deformation, light (in the form ofmuzzle flash) and a prominentmuzzle blast (which is responsible for theloud sound/concussive shock perceivable to bystanders and most of therecoil felt by the shooter, as well aspotentially deflecting the bullet), or as unusable kinetic energy transferred to otherejectabyproducts (e.g. unburnt powders, dislodgedfoulings).[citation needed]
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