What is "bourrelet"

n. A ring of softer material around the diameter of a shell of a gun, which acts as a sealant so that the propellent gases do not leak through the space between the shell and the barrel.

The bourrelet is the portion of an elongated artillery projectile having a very slightly smaller diameter than the interior diameter of the barrel through which that projectile is to be propelled. The bourrelet is typically precisely machined to a specified diameter after the remainder of the cast projectile body has been roughly machined to a smaller diameter. The bourrelet is normally forward of a rotating band or driving band near the aft end of the projectile. The rotating or 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. The bourrelet is not engraved by the rifling, but is supported by the lands between the rifling to align the long axis of the projectile with the axis of the artillery bore.