Downloaded from https://www.everyspec.com i"' sultant gas force applied to them may be transmitted bands are bands of relatively soft material which to the tube. Thus, tube and breech assembly act are welded, bonded, or mechanically attached to the as a unit in transmitting the resultant force to the projectile. Jackets may be considered as bands .strueture supporting the tfube. which Cover the hulletRs nompleta.ly. Either jacket, rotating band, or sabot is necessary to transmit to SC. BORE the projectile the angular accelerating force induced 7. The bore is formed by the inner surface of a by the rifling, and to serve as a seal to prevent pro- circular cylinder.
It is the accelerating tube for the pellant gases from escaping past the projectile. projectile. Early firearms were smooth bore and inherently inaccurate. Eventually, either by accident 9. Tapered bores (squeeze bores) have been used or design, it was discovered that spinning the pro- in the past as a mealis to increase muzzle velocity. jectile about the flight axis increased accuracy. An oversize bore at the chamber was gradually Several methods have been introduced to achieve tapered to a much smaller diameter near the muzzle. this rotation.
One method utilizes the behavior of The larger bore diameter exposed a larger pressure air impinging on canted fins attached to projectiles area to the propellant gases thereby increasing the such as projectiles fired from smooth bore mortars, projectile acceleration, The projectile relied on two Another method uses rifling to impart the rotation. rotating bands having oversized diameters to keep Rifling consists of splines which spiral along the it centered and in contact with the rifling. The bore surface. The raised portions are called lands, bands, spaced far enough apart to form an effective the spaces between them are called grooves.
The wheelbase, were squeezed inward toward, the base spiral may have a constant angle of twist, i.e., a of the projectile as it traveled along the tapered helix, or it may have a variable angle of twist con- bore. This concept was proved impractical and now forming to some exponential expression. As the has no more than historical significance. A more projectile moves through the bore, it turns with the logical use of the tapered bore is that practiced in rifling at an angular velocity proportional to the small arms.
In order to decrease its diameter slightly linear velocity and to the tangent of the angle of as the muzzle is approached, the bore is provided twist, with a shallow taper along its entire length. This ' ) design concept, better known as choke bore, has 8. Contact with the rifling is assured by having been adopted to compensate for the anticipated the diameter of the jacket of a bullet or the rotating erosion. In shotguns, choke bore is applied to a band of a larger projectile at least equal to the groove short distance at the muzzle to control the area of diameter. Then, just as the projectile starts to move, spread, or shot pattern. the band or jacket is forced into the rifling.
This process is called engraving and takes place at the D. MUZZLE beginning, or origin, of rifling. For recoilless guns, 10. The muzzle end of the tube serves as the at- rotating bands are preengraved to eliminate the tachment for front sights, blast deflectors, and large engraving forces which are undesirable in this muzzle brakes. Blast deflectors do not apply severe type weapon. Some mortar projectiles have a flaired loads to the tube. However, muzzle brakes develop skirt, or sabot, at the base which is pressed into the large forces which usually are transmitted to the rifling by the propellant gas pressure. Rotating tube by a threaded attachment near the muzzle. 3 S ~ .,nr .'ai -