Sabot

A sabot (UK: /sæˈboʊ, ˈsæboʊ/, US: /ˈseɪboʊ/) is a structural device used in firearm or cannon ammunition to keep a sub-caliber flight projectile, such as a relatively small bullet or arrow-type projectile, in the center of the barrel when fired, if the bullet has a significantly smaller diameter than the bore diameter of the weapon used. A sabot (UK: /sæˈboʊ, ˈsæboʊ/, US: /ˈseɪboʊ/) is a structural device used in firearm or cannon ammunition to keep a sub-caliber flight projectile, such as a relatively small bullet or arrow-type projectile, in the center of the barrel when fired, if the bullet has a significantly smaller diameter than the bore diameter of the weapon used. The sabot component in projectile design is more than simply the relatively thin, tough and deformable seal known as a driving band or obturation ring needed to trap propellant gases behind a projectile, and also keep the projectile centered in the barrel, when the outer shell of the projectile is only slightly smaller in diameter than the caliber of the barrel. Driving bands and obturators are used to seal these full-bore projectiles in the barrel because of manufacturing tolerances; there always exists some gap between the projectile outer diameter and the barrel inner diameter, usually a few thousandths of an inch; enough of a gap for high pressure gasses to slip by during firing. Driving bands and obturator rings are made from material that will deform and seal the barrel as the projectile is forced from the chamber into the barrel. Small caliber jacketed bullets do not normally employ driving bands or obturators because the jacket material, for example copper or gilding metal, is deformable enough to serve that function, and the bullet is made slightly larger than the barrel for that purpose, (see full metal jacket bullet and driving band). Sabots certainly use driving bands and obturators, because the same manufacturing tolerance issues exist when sealing the saboted projectile in the barrel, but the sabot itself is a more substantial structural component of the in-bore projectile configuration (Drysdale 1978). Refer to the two APFSDS (armor-piercing fin-stabilized discarding sabot) pictures on the right to see the substantial material nature of a sabot to fill the bore diameter around the sub-caliber arrow-type flight projectile, compared to the very small gap sealed by a driving band or obturator to mitigate what is known classically as windage. More detailed cutaways of the internal structural complexity of advanced APFSDS saboted long rod penetrator projectiles can be found at reference 2. The function of a sabot is to provide a larger bulkhead structure that fills the entire bore area between an intentionally designed sub-caliber flight projectile and the barrel, giving a larger surface area for propellant gasses to act upon than just the base of the smaller flight projectile (Drysdale 1978). Efficient aerodynamic design of a flight projectile does not always accommodate efficient interior ballistic design to achieve high muzzle velocity. This is especially true for arrow-type projectiles, which are long and thin for low drag efficiency, but too thin to shoot from a gun barrel of equal diameter to achieve high muzzle velocity. The physics of interior ballistics demonstrates why the use of a sabot is advantageous to achieve higher muzzle velocity with an arrow-type projectile. Propellant gasses generate high pressure, and the larger the base area that pressure acts upon the greater the net force on that surface. Force, pressure times area, provides an acceleration to the mass of the projectile. Therefore, for a given pressure and barrel diameter, a lighter projectile can be driven from a barrel to a higher muzzle velocity than a heavier projectile. However, a lighter projectile may not fit in the barrel, because it is too thin. To make up this difference in diameter, a properly designed sabot provides less parasitic mass than if the flight projectile were made full-bore, in particular providing dramatic improvement in muzzle velocity for APDS (Armor-piercing discarding sabot) and APFSDS ammunition. Seminal research on two important sabot configurations for long rod penetrators used in APFSDS ammunition, namely the 'saddle-back' and 'double-ramp' sabot was performed by the US Army Ballistics Research Laboratory during the development and improvement of modern 105mm and 120mm kinetic energy APFSDS penetrators(Drysdale 1978), permitted by the significant recent advancement in the computerized Finite element method in structural mechanics at that time; and now represents the existing fielded technology standard. (See for example the development of the M829 series of anti-tank projectiles beginning with the base model M829 in the early 1980s, to the recently fielded M829A4 model, employing ever longer 'double-ramp' sabots). Upon muzzle exit, the sabot is discarded, and the smaller flight projectile flies to the target with less drag resistance than a full-bore projectile. In this manner, very high velocity and slender, low drag projectiles can be fired more efficiently, (see external ballistics and terminal ballistics). Nevertheless, the weight of the sabot represents parasitic mass that must also be accelerated to muzzle velocity, but does not contribute to the terminal ballistics of the flight projectile. For this reason, great emphasis is placed on selecting strong yet lightweight structural materials for the sabot, and configuring the sabot geometry to efficiently employ these parasitic materials at minimum weight penalty (Drysdale 1978). The purpose of the sabot is to allow a smaller diameter flight projectile to be launched at greater muzzle velocity than if the flight projectile alone were fired from a gun of equal caliber (full-bore). Firing a smaller-sized projectile wrapped in a sabot raises the muzzle velocity of the projectile. Made of some lightweight material (usually high strength plastic in small caliber rifles, (see SLAP Saboted light armor penetrator), shotguns and muzzle loader ammunition; aluminium, steel, and carbon fiber reinforced plastic for modern anti-tank kinetic energy ammunition; and, in classic times, wood or papier-mâché – in muzzle loading cannons). The sabot usually consists of several longitudinal pieces held in place by the cartridge case, an obturator or driving band. When the projectile is fired, the sabot blocks the gas, provides significant structural support against launch acceleration, and carries the projectile down the barrel. When the sabot reaches the end of the barrel, the shock of hitting still air pulls the parts of the sabot away from the projectile, allowing the projectile to continue in flight. Modern sabots are made from high strength aluminum and graphite fiber reinforced epoxy. They are used primarily to fire long rods of very dense materials, such as tungsten heavy alloy and depleted uranium. (see for example the M829 series of anti-tank projectiles). Sabot-type shotgun slugs were marketed in the United States starting in about 1985. When used with a rifled slug barrel, they offer vastly improved accuracy compared to traditional shotgun slugs. They are now legal for hunting in most U.S. states.