The driving band is customarily made of a softer metal than the remainder of the artillery projectile and of a larger diameter than the bourrelet to seal the barrel and be engraved by the rifling to spin-stabilize the projectile in flight.
If the bourrelet diameter is not a close fit, the in-bore yaw angle will affect the stability of a projectile during flight.
However, excessive pressures may be generated within an artillery barrel attempting to fire a cast steel projectile with a bourrelet diameter lacking clearance to pass irregularities within the artillery barrel.
The United States Navy typically specified a bourrelet diameter 0.015 in (0.38 mm) smaller than nominal land diameter with a minus manufacturing tolerance so the average clearance was about 0.012 in (0.30 mm) to compensate for thermal expansion firing a projectile from a warm magazine through a cold barrel and to clear copper fouling from the driving bands of previously fired projectiles.
[2] In the case of a sub-caliber projectile, such as an armor-piercing round, a sabot surrounds the sub-caliber portion, with driving bands at the rear and the bourrelet supporting the front, aligning the armor-piercing portion with the center of the barrel of the gun.