Driving band

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 seal; this seal prevents the gases from blowing past the shell, and engages the barrel's rifling to spin-stabilize the shell.
Characteristics.
The shell is stabilized for yaw in the barrel by a smaller bourrelet band near the front of the projectile. This band keeps the projectile travelling straight in the bore, but doesn't engage the rifling.
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.
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.
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, which is now rapidly replacing older artillery worldwide.
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.