Cannelure

Cannelure (kan"ne*lur) , noun

[French, from canneler to groove.]

(Military) A groove in any cylinder; specif., a groove around the cylinder of an elongated bullet for small arms to contain a lubricant, or around the rotating band of a gun projectile to lessen the resistance offered to the rifling. Also, a groove around the base of a cartridge, where the extractor takes hold.