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A driving band or rotating band is a band of soft metal near the base of an artillery shell, often made of gilding metal, copper, or lead. When the shell is fired, the pressure of the propellant swages the metal into the rifling of the barrel and forms a gas seal; this seal prevents the gases from blowing past the shell and engages the barrel's rifling to spin-stabilize the shell.
As shell weight increases, it becomes more difficult to engineer a driving band that prevents propellant gases from either blowing past it, or blowing it off the shell. Tougher alloys like cupronickel may be used on major-caliber projectiles. Rotating band width of about one-third of the projectile caliber provides superior performance, but two narrower bands, separated by a short distance, have been used to conserve strategic metals in wartime. Each band is secured in a dovetailed notch machined into the projectile. Waved ridges, longitudinal nicks, or knurling is machined into the bottom of the notch to prevent the band from slipping around the projectile as the projectile accelerates down the gun barrel. The rotating band is made of a ring of slightly greater diameter than the projectile, slipped into position while thermally expanded, and pressed radially into place with a powerful hydraulic press.
The forward edge of the band may be conically tapered to fit into a coned seat at the start of the gun barrel rifling. The central portion of the band is roughly cylindrical with a diameter slightly larger than the groove diameter of the gun barrel to ensure a tight fit in gun barrels worn by firing previous projectiles. The rear portion of the band may include a flared skirt of even larger diameter in front of a groove to hold the skirt as it is compressed by barrel dimensions. The skirt is intended to provide a gas seal in the most heavily eroded portion of the bore near the powder chamber.
Gerald Bull worked extensively on ways to eliminate the driving band, leading to the development of his Extended Range, Full Bore ammunition using an inversion of the pre-cut rifling for his GC-45 howitzer.
Some weapons that operate at high rates of fire, such as the GAU-8 Avenger Gatling cannon, use plastic driving bands instead of soft metal. Using plastic as a swage material reduces wear on the barrel's rifling, and extends the life and average accuracy of the weapon.
In a small-arms rifle, the entire bullet is typically covered in copper or another soft alloy, making the entire bullet its own driving band.
During World War II, German ammunition sometimes used iron driving bands instead of copper due to material shortages. Porous iron bands were favored over solid ones.
Variations
Driving bands pre-cut for the rifling have been used for muzzle loaded weapons, e.g. some mortars.
Freely rotating bands can be used to reduce the spin imparted to the round as is preferable for HEAT warheads or fin-stabilised projectiles fired from general-purpose rifled barrels.
See also
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