Driving band
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The driving band is part of an artillery shell, a band of soft metal near the middle of the shell, typically made of copper or lead. When the shell is fired the pressure of the propellant squeezes the metal into the rifling of the barrel, both providing for a seal as well as providing a driving surface to spin the shell. In a rifle, the entire bullet is typically covered in copper, as so the entire bullet is its own driving band.
One downside to the driving band is that it must be placed at the widest point on the shell, and also near the balance point. This is not nessessarily the best point from an aerodynamic perspective, at high supersonic speeds the widest point should be far to the rear of the balance point. Another downside is that with increasing shell weight the driving band becomes more difficult to build properly. Instead of the propellant gasses driving the shell up the barrel, it can simply blow the driving band right off the shell.
Gerald Bull worked extensively on ways to eliminate the driving band, leading to the development of his Extended Range, Full Bore ammunition for his GC-45 howitzer, which is now rapidly replacing older artillery world-wide.